importTracker: Fix error for undefined importClause

tests/cases/fourslash/findAllRefsNoImportClause.ts

@@ -0,0 +1,11 @@
+/// <reference path="fourslash.ts" />
+
+// https://github.com/Microsoft/TypeScript/issues/15452
+
+// @Filename: /a.ts
+////export const [|{| "isWriteAccess": true, "isDefinition": true |}x|] = 0;
+
+// @Filename: /b.ts
+////import "./a";
+
+verify.singleReferenceGroup("const x: 0");