calc: Improve handling of invalid 'calc-string-maximum-character'

Previously, if 'calc-string-maximum-character' wasn't a valid
character 'math-vector-is-string' would throw an error in the
comparison, leading to an incomplete display of the stack and a
cryptic error message.  Instead, have 'math-vector-is-string'
return nil, which effectively disables the display of strings.
Refines feature introduced in bug#78528.
* doc/misc/calc.texi (Customizing Calc): Update description of
behavior for invalid 'calc-string-maximum-character'.
* lisp/calc/calccomp.el (math-vector-is-string): Return nil when
'calc-string-maximum-character' doesn't represent a character.
* test/lisp/calc/calc-tests.el (calc-math-vector-is-string): Correct
and simplify tests.

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