fix(parser): handle empty strings

modules/change_detection/src/parser/ast.js

@@ -18,6 +18,16 @@ export class AST {
   }
 }
 
+export class EmptyExpr extends AST {
+  eval(context) {
+    return null;
+  }
+
+  visit(visitor, args) {
+    //do nothing
+  }
+}
+
 export class ImplicitReceiver extends AST {
   eval(context) {
     return context;

modules/change_detection/src/parser/parser.js

@@ -5,6 +5,7 @@ import {Lexer, EOF, Token, $PERIOD, $COLON, $SEMICOLON, $LBRACKET, $RBRACKET,
 import {reflector, Reflector} from 'reflection/reflection';
 import {
   AST,
+  EmptyExpr,
   ImplicitReceiver,
   AccessMember,
   LiteralPrimitive,
@@ -146,7 +147,9 @@ class _ParseAST {
         this.error(`Unexpected token '${this.next}'`);
       }
     }
-    return exprs.length == 1 ? exprs[0] : new Chain(exprs);
+    if (exprs.length == 0) return new EmptyExpr();
+    if (exprs.length == 1) return exprs[0];
+    return new Chain(exprs);
   }
 
   parseFormatter() {