KShell:tildeExpand: Return filename if homedir is empty

Akseli Lahtinenrequested to merge
work/akselmo/fix-tilde-expand into master

Often cache and automatic backup files are saved with ~ prefix. Our tilde-expand method returns empty in cases where the ~file does not match an username.

In those cases, just return the filename instead.

Also updated autotest.

BUG: 317513

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