Prevent BeforeInputPlugin from returning [null, null] (#11848)

The BeforeInputPlugin dispatches null elements in an array if
composition or beforeInput events are not extracted. This causes a
an extra array allocation, but more importantly creates null states in
later event dispatch methods that are annoying to account for.

This commit makes it so that BeforeInputPlugin never returns a null
element inside an array.

packages/react-dom/src/events/BeforeInputEventPlugin.js

@@ -453,20 +453,29 @@ const BeforeInputEventPlugin = {
     nativeEvent,
     nativeEventTarget,
   ) {
-    return [
-      extractCompositionEvent(
-        topLevelType,
-        targetInst,
-        nativeEvent,
-        nativeEventTarget,
-      ),
-      extractBeforeInputEvent(
-        topLevelType,
-        targetInst,
-        nativeEvent,
-        nativeEventTarget,
-      ),
-    ];
+    const composition = extractCompositionEvent(
+      topLevelType,
+      targetInst,
+      nativeEvent,
+      nativeEventTarget,
+    );
+
+    const beforeInput = extractBeforeInputEvent(
+      topLevelType,
+      targetInst,
+      nativeEvent,
+      nativeEventTarget,
+    );
+
+    if (composition === null) {
+      return beforeInput;
+    }
+
+    if (beforeInput === null) {
+      return composition;
+    }
+
+    return [composition, beforeInput];
   },
 };
 

packages/react-dom/src/events/__tests__/BeforeInputEventPlugin-test.internal.js

@@ -76,7 +76,12 @@ describe('BeforeInputEventPlugin', function() {
     return function() {
       const newArgs = [node].concat(Array.prototype.slice.call(arguments));
       const newEvents = extract.apply(this, newArgs);
-      Array.prototype.push.apply(events, newEvents);
+
+      if (Array.isArray(newEvents)) {
+        Array.prototype.push.apply(events, newEvents);
+      } else if (newEvents) {
+        events.push(newEvents);
+      }
     };
   }
 
@@ -156,32 +161,17 @@ describe('BeforeInputEventPlugin', function() {
   // keyUp.
   const Expected_Webkit = () => [
     {type: ModuleCache.SyntheticCompositionEvent, data: {}},
-    {type: null},
-    {type: null},
     {type: ModuleCache.SyntheticInputEvent, data: {data: 'A'}},
-    {type: null},
-    {type: null}, // textinput of A
-    {type: null},
-    {type: null}, // keyUp of 65
-    {type: null},
+    // textinput of A
+    // keyUp of 65
     {type: ModuleCache.SyntheticInputEvent, data: {data: 'abc'}},
-    {type: null},
-    {type: null}, // textinput of abc
-    {type: null},
-    {type: null}, // keyUp of 32
-    {type: null},
+    // textinput of abc
+    // keyUp of 32
     {type: ModuleCache.SyntheticInputEvent, data: {data: 'xyz'}},
-    {type: null},
-    {type: null}, // textinput of xyz
-    {type: null},
-    {type: null}, // keyUp of 32
+    // textinput of xyz
+    // keyUp of 32
     {type: ModuleCache.SyntheticCompositionEvent, data: {data: 'Hello'}},
-    {type: null},
-
     // Emoji test
-    {type: null},
-    {type: null},
-    {type: null},
     {type: ModuleCache.SyntheticInputEvent, data: {data: '\uD83D\uDE0A'}},
   ];
 
@@ -191,39 +181,22 @@ describe('BeforeInputEventPlugin', function() {
   // element, not event data.
   const Expected_IE11 = () => [
     {type: ModuleCache.SyntheticCompositionEvent, data: {}},
-    {type: null},
-    {type: null},
-    {type: null}, // textInput of A
-    {type: null},
-    {type: null}, // textinput of A
-    {type: null},
-    {type: null}, // keyUp of 65
-    {type: null},
-    {type: null}, // textInput of abc
-    {type: null},
-    {type: null}, // textinput of abc
-
+    // textInput of A
+    // textinput of A
+    // keyUp of 65
+    // textInput of abc
+    // textinput of abc
     // fallbackData should NOT be set at keyUp with any of END_KEYCODES
-    {type: null},
-    {type: null}, // keyUp of 32
-
-    {type: null},
-    {type: null}, // textInput of xyz
-    {type: null},
-    {type: null}, // textinput of xyz
-    {type: null},
-    {type: null}, // keyUp of 32
-
+    // keyUp of 32
+    // textInput of xyz
+    // textinput of xyz
+    // keyUp of 32
     // fallbackData is retrieved from the element, which is XYZ,
     // at a time of compositionend
     {type: ModuleCache.SyntheticCompositionEvent, data: {}},
     {type: ModuleCache.SyntheticInputEvent, data: {data: 'XYZ'}},
-
     // Emoji test
-    {type: null},
     {type: ModuleCache.SyntheticInputEvent, data: {data: '\uD83D\uDE0A'}},
-    {type: null},
-    {type: null},
   ];
 
   function TestEditableReactComponent(Emulator, Scenario, ExpectedResult) {