Fix off-by-one error preventing empty string literals (#361)

packages/adl/compiler/scanner.ts

@@ -444,11 +444,10 @@ export function createScanner(source: string | SourceFile, onError = throwOnErro
     if (tripleQuoted) {
       tokenFlags |= TokenFlags.TripleQuoted;
       quoteLength = 3;
+      position += 2;
       closing = '"""';
     }
 
-    position += quoteLength;
-
     scanUntil(
       (ch) => {
         if (isEscaping) {

packages/adl/test/test-scanner.ts

@@ -145,6 +145,10 @@ describe("scanner", () => {
     strictEqual(scanner.getTokenValue(), expectedValue);
   }
 
+  it("scans empty strings", () => {
+    scanString('""', "");
+  });
+
   it("scans strings single-line strings with escape sequences", () => {
     scanString('"Hello world \\r\\n \\t \\" \\\\ !"', 'Hello world \r\n \t " \\ !');
   });