[BUGFIX] Cached class loader misses are not considered

The class loader caches if it doesn't find a class name
for subsequent calls. But currently it only checks for
NULL results from the cache and not the empty strings
returned from the cache and thus tries to resolve the
non existing class name over and over again.

The fix converts the empty string to a NULL and
changes the semantics of NULL to FALSE in the class
loader.

Fixes: #56933
Releases: 6.2
Change-Id: I4cd1a18b4a4496c287a937d6d9882ae21d3ee9a3
Reviewed-on: https://review.typo3.org/28419
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Markus Klein
Tested-by: Markus Klein