Avoid useless warning if purpose was not provided

bin/mkexec

 $cmds->{editor}->{test} = "editor --version 2>/dev/null";
 $cmds->{editor}->{run}  = "editor '%s'";
 
-unless (exists $templates->{$type}->{$purpose}) {
+unless ($purpose eq ""
+        or exists $templates->{$type}->{$purpose}) {
     warn "undefined purpose '$purpose' for type '$type'\n";
     $purpose = "";
 }