Don't throw on an undefined partial

Fixes dependents/node-detective-cjs#10
Refs pahen/madge#197

index.js

@@ -242,6 +242,12 @@ function commonJSLookup({dependency, filename, directory, nodeModulesConfig}) {
   if (!resolve) {
     resolve = require('resolve');
   }
+
+  if (!dependency) {
+    debug('blank dependency given. Returning early.');
+    return '';
+  }
+
   // Need to resolve partials within the directory of the module, not filing-cabinet
   const moduleLookupDir = path.join(directory, 'node_modules');
 

test/mockedJSFiles.js

@@ -3,7 +3,8 @@ module.exports = {
     'es6': {
       'foo.js': 'import bar from "./bar";',
       'foo.jsx': 'import React from "react"; export default () => { return (<div></div>); }',
-      'bar.js': 'export default function() {};'
+      'bar.js': 'export default function() {};',
+      'lazy.js': 'localizedModule(app, "modulename", locale => import(`modulename/locales/${locale}`));'
     },
     'cjs': {
       'foo.js': 'module.exports = 1;',

test/test.js

@@ -153,6 +153,30 @@ describe('filing-cabinet', function() {
         assert.equal(result, expected);
         spy.restore();
       });
+
+      describe('when given a lazy import with interpolation', function() {
+        it('does not throw', function() {
+          assert.doesNotThrow(() => {
+            cabinet({
+              partial: '`modulename/locales/${locale}`',
+              filename: 'js/es6/lazy.js',
+              directory: 'js/es6/'
+            });
+          });
+        });
+      });
+
+      describe('when given an undefined dependency', function() {
+        it('does not throw', function() {
+          assert.doesNotThrow(() => {
+            cabinet({
+              partial: undefined,
+              filename: 'js/es6/lazy.js',
+              directory: 'js/es6/'
+            });
+          });
+        });
+      });
     });
 
     describe('jsx', function() {