patch 9.1.1121: Enter does not insert newline with "noselect"

patch 9.1.1121: Enter does not insert newline with "noselect"

Problem:  Enter does not insert newline with "noselect" when the pum is
          visible (lifepillar)
Solution: When Enter is pressed and no complete-item is selected,
          ins_compl_prep returns false, and the edit function continues
          processing Enter to insert a new line. (glepnir)

fixes: #1653
closes: #16653

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>