Remove empty expression

Running unit tests tmain and units via 'make check' the empty expression
.{0} from the matching group in parsers/robot.c was causing FreeBSD libc
regcomp to produce an error: empty (sub)expression.

tmain would fail for some checks while units would fail for robots checks.

Removing the emptry expression .{0} "match anything exactly zero times"
fixes this issue and all expected tests pass for both tmain and units.

Testing the same change, removing the expression .{0} from the
matching group, on centos7 does not appear to have any regessions.

This reverts the following commit
universal-ctags@81a4593

That was created for the following issue:
universal-ctags#1571 (comment)

It's possible there's some regression for macOS however macOS on Travis
doesn't seem to have any issues.

parsers/robot.c

@@ -144,7 +144,7 @@ static void initialize (const langType language)
 
     addLanguageCallbackRegex (
 		language,
-		"(^([A-Za-z0-9]+|\\$\\{[_A-Za-z0-9][' _A-Za-z0-9]*(:([^}]|\\\\|.{0})+)*\\})([${}' _]([-_$A-Za-z0-9]+|\\{[_A-Za-z0-9][' _A-Za-z0-9]*(:([^}]|\\\\|.{0})+)*\\})+)*)",
+		"(^([A-Za-z0-9]+|\\$\\{[_A-Za-z0-9][' _A-Za-z0-9]*(:([^}]|\\\\)+)*\\})([${}' _]([-_$A-Za-z0-9]+|\\{[_A-Za-z0-9][' _A-Za-z0-9]*(:([^}]|\\\\)+)*\\})+)*)",
 		"{exclusive}", tagKeywordsAndTestCases, NULL, NULL);
 
     addLanguageCallbackRegex (language, "^[$@]\\{([_A-Za-z0-9][' _A-Za-z0-9]+)\\}  [ ]*.+",