This makes the parser cache resilient to encountering string values
where it is currently expecting to get a ParserOutput objerct from the
underlying cache.
This provides forward compatibility with a switch to JSON based caching:
If we have to switch back after writing JSON to the cache for a while,
ParserCache would simply ignore the respective entries, rather than
causing fatal errors.
Bug: T263579
Change-Id: Iaed582097ab2d05edb4b99a738ac39c530fd63c1
ff07253be5