Subcommand auto-completion misses trailing space

### Description:

When auto-completing partially filled subcommands with ``, the subcommand is completed, but in case of unique subcommand options, no trailing space is added after the subcommand.

### Steps to reproduce:

1. Write `task gh` and press ``
2. Write `task ann` and press ``

### Expected results:

1. Command expands to `task ghistory`, with no trailing space, given that subcommand completion has multiple options (`ghistory.annual`, `ghistory.daily`, etc) *(Actually, a trailing period `.` could also be added, but this is a rather niche use-case.)*
2. Command expands to `task annotate `, with trailing space, given that there are no multiple subcommand options available.

### Actual results:

1. Command expands to `task ghistory`, with no trailing space, as expected.
2. Command expands to `task annotate`, with no trailing space, and user has to hit space before writing the `` text.

### Remarks:

I understand there are subcommands or filters that can be compounded (`description.contains`, `due.before`, etc), where adding a trailing space is not desired, but for the above example and some other subcommands it doesn't seem to be the case.

### Version:

3.4.1.