report error if module name is empty

src/compiler/checker.ts

@@ -965,7 +965,7 @@ namespace ts {
             // Escape the name in the "require(...)" clause to ensure we find the right symbol.
             let moduleName = escapeIdentifier(moduleReferenceLiteral.text);
 
-            if (!moduleName) {
+            if (moduleName === undefined) {
                 return;
             }
             let isRelative = isExternalModuleNameRelative(moduleName);

tests/baselines/reference/emptyModuleName.errors.txt

@@ -0,0 +1,9 @@
+tests/cases/compiler/emptyModuleName.ts(1,20): error TS2307: Cannot find module ''.
+
+
+==== tests/cases/compiler/emptyModuleName.ts (1 errors) ====
+    import * as A from "";
+                       ~~
+!!! error TS2307: Cannot find module ''.
+    class B extends A {
+    }

tests/baselines/reference/emptyModuleName.js

@@ -0,0 +1,19 @@
+//// [emptyModuleName.ts]
+import * as A from "";
+class B extends A {
+}
+
+//// [emptyModuleName.js]
+var __extends = (this && this.__extends) || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+};
+var A = require("");
+var B = (function (_super) {
+    __extends(B, _super);
+    function B() {
+        _super.apply(this, arguments);
+    }
+    return B;
+})(A);

tests/cases/compiler/emptyModuleName.ts

@@ -0,0 +1,4 @@
+// @module: commonjs
+import * as A from "";
+class B extends A {
+}