feat: allow empty value for /pgf/arrow keys/fill

[hmenke]

Motivation for this change

Fixes https://github.com/pgf-tikz/pgf/issues/1348

Checklist

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• [x] Code changes are licensed under GPLv2 + LPPLv1.3c
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[muzimuzhi]

@hmenke Do you think this needs a new test?