add EMPTY keyword

Author: Andrin Meier

Rubberduck.Parsing/Grammar/VBA.g4

@@ -559,7 +559,7 @@ letterrange : certainIdentifier (WS? MINUS WS? certainIdentifier)?;
 
 lineLabel : ambiguousIdentifier ':';
 
-literal : HEXLITERAL | OCTLITERAL | DATELITERAL | DOUBLELITERAL | INTEGERLITERAL | SHORTLITERAL | STRINGLITERAL | TRUE | FALSE | NOTHING | NULL;
+literal : HEXLITERAL | OCTLITERAL | DATELITERAL | DOUBLELITERAL | INTEGERLITERAL | SHORTLITERAL | STRINGLITERAL | TRUE | FALSE | NOTHING | NULL | EMPTY;
 
 type : (baseType | complexType) (WS? LPAREN WS? RPAREN)?;
 
@@ -650,6 +650,7 @@ DOUBLE : D O U B L E;
 EACH : E A C H;
 ELSE : E L S E;
 ELSEIF : E L S E I F;
+EMPTY : E M P T Y;
 END_ENUM : E N D WS E N U M;
 END_FUNCTION : E N D WS F U N C T I O N;
 END_IF : E N D WS I F;