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

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>