Revert "type: fix undefined index in ds_names[$source] before empty check"

From the docs:
"No warning is generated if the variable does not exist. That means empty() is
essentially the concise equivalent to !isset($var) || $var == false."

This reverts commit f2a4593558b3a3ec696e9cc1de93fa898b9521bb.

Reported-By: Benoit Monin (https://github.com/pommi/CGP/commit/f2a4593)