[FIRRTL] Fix FModuleOp::erasePorts failing on empty attr arrays

As an optimization, we use empty array attributes on module-like ops
where none of the ports have an attribute set. The code in `erasePorts`
assumes that all arrays have the same number of entries as the module
has ports.

This adds a special case that just passes through empty arrays (the
optimized case).

lib/Dialect/FIRRTL/FIRRTLOps.cpp

@@ -50,6 +50,11 @@ removeElementsAtIndices(ArrayRef<T> input, ArrayRef<unsigned> indicesToDrop) {
   }
 #endif
 
+  // If the input is empty (which is an optimization we do for certain array
+  // attributes), simply return an empty vector.
+  if (input.empty())
+    return {};
+
   // Copy over the live chunks.
   size_t lastCopied = 0;
   SmallVector<T> result;