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 <[email protected]>

Author: Jorenar

gdb/python/lib/gdb/dap/completions.py

@@ -39,8 +39,11 @@ def completions(
         line = 1
     else:
         line = import_line(line)
-    text = text.splitlines()[line - 1]
-    text = text[: column - 1]
+    if text:
+        text = text.splitlines()[line - 1]
+        text = text[: column - 1]
+    else:
+        text = ""
     mi_result = exec_mi_and_log("-complete", text)
     result = []
     completion = None