Check for the "no output" case in filter_program_output.

E.g. for git command output when we haven't quite cloned the module for the
first time you're likely to get undef for the output (since the command will
just fail).  Check for that.

kdesrc-build

@@ -4822,7 +4822,7 @@ sub git_commit_id
         undef, # No filter
         qw/git --git-dir/, $gitdir, 'rev-parse', $commit,
     );
-    chomp $id;
+    chomp $id if $id;
 
     return $id;
 }