Fix linter cases where specific xpath expression problems are not reported

[pgfearo]

• then must immediately follow the parenthesised condition: if (1) eq 1 then 1 else 2"
• () is not valid after if token
• string-literals: a single-quote escaped by another single quote must be followed by another single-quote
• built-in functions and true() and false() must not be followed by () - current() is OK
• function() without following curly brace is not valid

[pgfearo]

Included in Release 1.5.13