Fix GH-24256: fclose() returns false for wrappers without stream_flush()

[lazerg]

Since c07cc85, fclose(), file_put_contents() and copy() return the result of the flush on close. php_userstreamop_flush() returns -1 when the wrapper has no stream_flush() method, so every write to such a wrapper now makes these functions return false. During close, a missing stream_flush() now counts as nothing to flush. An explicit fflush() still returns false for these wrappers, as SplFileObject_fflush_basic_001.phpt expects.

Fixes #24256