avoid file_get_contents error if file does not exist any more

Our tests generate some PHP files which are executed and later deleted.
PHPCodeCoverage then fails with an error because it tries to open it with file_get_contents.
(The file is created in a directory that is in filter/blacklist but that does not seem to help, I did not investigate further).

The extra `file_exists` call may be too costly, I do not know if you will accept the change, but I will propose it just in case.