revset: fix crash on empty sort key
Make it noop as before 2188f170f5b6. We could change it to an error, but
allowing empty key makes some sense for scripting that builds a key string
programmatically.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing:
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See https://mercurial-scm.org/ for detailed installation
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