convert: treat an empty string for clean/smudge filters as "cat"

Once a lower-priority configuration file defines a clean or smudge filter, there is no convenient way to override it to produce as-is output. Even though the configuration mechanism implements "the last one wins" semantics, you cannot set them to an empty string and expect them to work, as apply_filter() would try to run the empty string as an external command and fail. The conversion is not done, but the function would still report a failure to convert. Even though resetting the variable to "cat" (i.e. pass the data back as-is and report success) is an obvious and a viable way to solve this, it is wasteful to spawn an external process just as a workaround. Instead, teach apply_filter() to treat an empty string as a no-op filter that always returns successfully its input as-is without conversion. Signed-off-by: Lars Schneider <larsxschneider@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>