Return a non-printing sequence for no context separators

Function symbols_get_context_separator() returns the symbol
separator for the language, but some languages do not have
symbol context separators, for example markup languages like
Asciidoc.  To prevent the symbols pane wrongly detecting and
acting on a valid character sequence as a separator, return a
non-printing character which should not occur.

Options "" and NULL not used as they break some code and would
need an ABI bump.

Other languages can be added as they are identified.