patch 9.1.1150: :hi completion may complete to wrong value

patch 9.1.1150: :hi completion may complete to wrong value

Problem:  :highlight auto-complettion has a minor bug where an existing
          highlight group with a cterm color being unset would result in
          it being auto-completed to -1 in cmdline which is invalid.

Solution: Correctly check for whether an int value is set to non-zero
          before retrieving the existing value for auto-complete. Also
          do this for attr/string values as they previously worked only
          by accident (Yee Cheng Chin).

closes: #16726

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>