Fix nested completer which got broken after previous 'get_word_before_cursor' fix.

Author: jonathanslenders

src/prompt_toolkit/document.py

@@ -457,10 +457,11 @@ def get_word_before_cursor(
     def _is_word_before_cursor_complete(
         self, WORD: bool = False, pattern: Pattern[str] | None = None
     ) -> bool:
-        if not self.text_before_cursor == "" or self.text_before_cursor[-1:].isspace():
+        if self.text_before_cursor == "" or self.text_before_cursor[-1:].isspace():
             return True
         if pattern:
             return self.find_start_of_previous_word(WORD=WORD, pattern=pattern) is None
+        return False
 
     def find_start_of_previous_word(
         self, count: int = 1, WORD: bool = False, pattern: Pattern[str] | None = None