Raise TextFSMTemplateError for a Value line with no regex

[arpitjain099]

A Value line that carries options and a name but no regex, which is an easy typo to make when the parentheses get dropped, crashes with a raw IndexError instead of the library's own template error.

>>> import io, textfsm
>>> textfsm.TextFSM(io.StringIO("Value Required foo\n\nStart\n  ^$$ -> Record\n"))
IndexError: string index out of range

self.regex ends up as "", and the check right after it does self.regex[0] before anything validates that the string is non-empty. Since IndexError is not TextFSMTemplateError, it slips past the handler in _ParseFSMVariables, so callers that catch template errors to report a bad template get a builtin exception instead.

An empty regex genuinely is not contained within a () pair, so the existing check and message cover it once the emptiness is tested first. Regexes of length one such as ( or ) already short-circuit on the first two comparisons, so the empty string was the only input that reached [-2] on a too-short value.

Added the missing-regex case to testFSMValue (through TextFSMValue.Parse) and to the malformed-variable block of testParseFSMVariables (through the template path). Both fail with IndexError before the change. Full suite is 33 passed after it.