patch 9.1.1383: completion: 'isexpand' option does not handle space char correct

patch 9.1.1383: completion: 'isexpand' option does not handle space char correct

Problem:  When a space character is used as a trigger in 'isexpand' option
          it doesn't get recognized because skip_to_option_part() skips
          spaces after a comma, treating them as option separators
          rather than option value (after v9.1.1341)
Solution: manually set the part to a space character (glepnir).

closes: #17305

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