completion: add space after config variable names also in Bash 3

In be6444d1ca (completion: bash: add correct suffix in variables, 2021-08-16), __git_complete_config_variable_name was changed to use "${sfx- }" instead of "$sfx" as the fourth argument of _gitcomp_nl and _gitcomp_nl_append, such that this argument evaluates to a space if sfx is unset. This was to ensure that e.g. git config branch.autoSetupMe[TAB] correctly completes to 'branch.autoSetupMerge ' with the trailing space. This commits notes that the fix only works in Bash 4 because in Bash 3 the 'local sfx' construct at the beginning of __git_complete_config_variable_name creates an empty string. Make the fix also work for Bash 3 by using the "unset or null' parameter expansion syntax ("${sfx:- }"), such that the parameter is also expanded to a space if it is set but null, as is the behaviour of 'local sfx' in Bash 3. Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>