Fix empty string literal error

Author: Noah Chen

Diff for: test/programs/string-literals-inline/main.ts

@@ -1,4 +1,4 @@
 class MyObject {
-    foo: "ok" | "fail" | "abort";
+    foo: "ok" | "fail" | "abort" | "";
     bar: "ok" | "fail" | "abort" | string;
 }

Diff for: test/programs/string-literals-inline/schema.json

@@ -6,6 +6,7 @@
         },
         "foo": {
             "enum": [
+                "",
                 "abort",
                 "fail",
                 "ok"

Diff for: test/programs/string-literals/main.ts

@@ -1,4 +1,4 @@
-type result = "ok" | "fail" | "abort";
+type result = "ok" | "fail" | "abort" | "";
 
 class MyObject {
     foo: result;

Diff for: test/programs/string-literals/schema.json

@@ -3,6 +3,7 @@
     "definitions": {
         "result": {
             "enum": [
+                "",
                 "abort",
                 "fail",
                 "ok"

Diff for: typescript-json-schema.ts

@@ -236,15 +236,18 @@ export class JsonSchemaGenerator {
     }
 
     private extractLiteralValue(typ: ts.Type): PrimitiveType | undefined {
+        let str = (<ts.LiteralType>typ).value;
+        if (str === undefined) {
+            str = (typ as any).text;
+        }
         if (typ.flags & ts.TypeFlags.EnumLiteral) {
             // or .text for old TS
-            let str = (<ts.LiteralType>typ).value || (typ as any).text;
-            let num = parseFloat(str);
+            let num = parseFloat(str as string);
             return isNaN(num) ? str : num;
         } else if (typ.flags & ts.TypeFlags.StringLiteral) {
-            return (<ts.LiteralType>typ).value || (typ as any).text;
+            return str;
         } else if (typ.flags & ts.TypeFlags.NumberLiteral) {
-            return parseFloat((<ts.LiteralType>typ).value || (typ as any).text);
+            return parseFloat(str as string);
         } else if (typ.flags & ts.TypeFlags.BooleanLiteral) {
             return (typ as any).intrinsicName === "true";
         }