SPIGOT-6303: Interpret text that Gson considers to be an empty JSON document as legacy text.
For example, this affects empty Strings, and Strings that consist only
of whitespace. In lenient mode this would also affect text that contains
comments, but that is not the case currently.
Minecraft's component parser (i.e. Gson) produces null for these texts
instead of throwing a parse exception. By interpreting this text as
legacy text we correctly create text components for them that contain
their contents.
