Change ABIEncoderV1 to not pad empty strings

libsolidity/codegen/CompilerUtils.cpp

@@ -531,10 +531,15 @@ void CompilerUtils::encodeToMemory(
 			if (_givenTypes[i]->category() == Type::Category::StringLiteral)
 			{
 				auto const& strType = dynamic_cast<StringLiteralType const&>(*_givenTypes[i]);
-				m_context << u256(strType.value().size());
+				auto const size = strType.value().size();
+				m_context << u256(size);
 				storeInMemoryDynamic(*TypeProvider::uint256(), true);
 				// stack: ... <end_of_mem'>
-				storeInMemoryDynamic(strType, _padToWordBoundaries);
+
+				// Do not output empty padding for zero-length strings.
+				// TODO: handle this in storeInMemoryDynamic
+				if (size != 0)
+					storeInMemoryDynamic(strType, _padToWordBoundaries);
 			}
 			else
 			{

libsolidity/codegen/CompilerUtils.h

@@ -104,12 +104,13 @@ public:
 	/// Stores a 256 bit integer from stack in memory.
 	/// @param _offset offset in memory
 	void storeInMemory(unsigned _offset);
+
 	/// Dynamic version of @see storeInMemory, expects the memory offset below the value on the stack
 	/// and also updates that. For reference types, only copies the data pointer. Fails for
-	/// non-memory-references.
+	/// non-memory-references. For string literals no value is available on the stack.
 	/// @param _padToWords if true, adds zeros to pad to multiple of 32 bytes. Array elements
+	///                    are always padded (except for byte arrays), regardless of this parameter.
 	/// @param _cleanup if true, adds code to cleanup the value before storing it.
-	/// are always padded (except for byte arrays), regardless of this parameter.
 	/// Stack pre: memory_offset value...
 	/// Stack post: (memory_offset+length)
 	void storeInMemoryDynamic(Type const& _type, bool _padToWords = true, bool _cleanup = true);