AK: Don't call memcpy with null argument in ByteBuffer::copy()

This was happening in TestBase64.test_decode, while copying an empty
string.

AK/ByteBuffer.h

@@ -74,7 +74,8 @@ public:
     [[nodiscard]] static ByteBuffer copy(void const* data, size_t size)
     {
         auto buffer = create_uninitialized(size);
-        __builtin_memcpy(buffer.data(), data, size);
+        if (size != 0)
+            __builtin_memcpy(buffer.data(), data, size);
         return buffer;
     }