basic/string-util: tweak strverscmp_improved() for some corner cases

basic/string-util: tweak strverscmp_improved() for some corner cases

So far we had the rule that '' == '', '0_' == '0', but '_' > ''. This means
that the general rule that strings are compared iteratively, and each
segment that compares equal can be dropped and the comparison resumes at
the following characters wasn't true in such cases. Similarly, '0~' < '0',
but after dropping the common segment, '~' > ''.

The special handling of empty strings is dropped, and '_' == '' and
'~' < ''.