sysdir: fix formatting error message with NULL string

When trying to the template dir, we pass in a `NULL` pointer for the
filename. That's perfectly fine, but if we're failing to find the
template directory then we'll creat an error message with the `NULL`
pointer passed in.

Fix the issue by setting different error messages based on whether the
filename is given or not. This even makes sense, as we're not searching
for a file in case we have no `name`, but for a directory. So the error
would've been misleading anyway.

src/sysdir.c

@@ -298,8 +298,11 @@ static int git_sysdir_find_in_dirlist(
 	}
 
 done:
-	git_buf_dispose(path);
+	if (name)
 		git_error_set(GIT_ERROR_OS, "the %s file '%s' doesn't exist", label, name);
+	else
+		git_error_set(GIT_ERROR_OS, "the %s directory doesn't exist", label);
+	git_buf_dispose(path);
 	return GIT_ENOTFOUND;
 }