Allow empty blocks around simple inverses

Previously, the parser required at least one character of whitespace
to properly interpret empty blocks around simple inverses, which was
non-intuitive and inconsistent with empty block parsing.

Update the parser to allow empty blocks around simple inverses.

src/handlebars.yy

@@ -7,8 +7,11 @@ root
   ;
 
 program
-  : statements simpleInverse statements { $$ = new yy.ProgramNode($1, $3); }
+  : simpleInverse statements { $$ = new yy.ProgramNode([], $2); }
+  | statements simpleInverse statements { $$ = new yy.ProgramNode($1, $3); }
+  | statements simpleInverse { $$ = new yy.ProgramNode($1, []); }
   | statements { $$ = new yy.ProgramNode($1); }
+  | simpleInverse { $$ = new yy.ProgramNode([], []); }
   | "" { $$ = new yy.ProgramNode([]); }
   ;