gdb/dap: fix completion request for empty strings

When DAP completion requests receives empty string to complete,
the script crashes due trying to access element -1 from list
being a result of `text.splitlines()` (which for `text == ""`
evaluates into empty list).

This patch adds simple check if `text` is populated, and when it
is not, skips transformations and assigns correct result directly.

Approved-By: Tom Tromey <tom@tromey.com>