bug#54639: 26.1; (not (string-empty-p nil)) returns true

[Ernesto Alfonso]

The expression (not (string-empty-p nil)) returns true. This is very
conuterintuitive: nil is as "empty" a value as there is! Some of the
more reasonable alternatives I can think of in order of preference are:

1. (string-empty-p nil) simply returns t
2. (string-empty-p nil) raises a type error
3. subr-x.el provides a separate string-null-or-empty-p function



In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-09-22, modified by Debian built on x86-grnet-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description:     Debian GNU/Linux 10 (buster)

Recent messages:
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killed: (string-empty-p sip-autosend-message)
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