gdb: test to check one aspect of the linespec parsing code

While working on the fix for PR cli/28665 (see previous couple of
commits), I was playing with making a change in the linespec parsing
code.  Specifically, I was thinking about whether the spec_string for
LINESPEC_LOCATION locations should ever be nullptr.

I made a change to prevent the spec_string from ever being nullptr,
tested gdb, and saw no regressions.

However, as part of this work I was reviewing how the breakpoint code
handles this case (spec_string being nullptr), and spotted that in
parse_breakpoint_sals the nullptr case is specifically handled, so
changing this should have caused a regression.  But I didn't see one.

So, this commit adds a comment in location.c mentioning that the
nullptr case is (a) not an oversight, and (b) is required.  Then I add
a new test to gdb.base/break.exp that ensures a change in this area
will cause a regression.

This test passes on current gdb, but with my modified (and broken)
gdb, the test would fail.