fix(autocomplete): handle undefined searchText

If the autocomplete's bound searchText was set to undefined,
it would throw an error.

Fix by ensuring that the searchText will always be an empty
string when performing the `handleQuery()` method.

Fixes #5162. Fixes #5393. References #5344. Closes #5445.

src/components/autocomplete/autocomplete.spec.js

@@ -582,6 +582,38 @@ describe('<md-autocomplete>', function() {
     }));
   });
 
+  describe('when required', function() {
+    it('properly handles md-min-length="0" and undefined searchText', function() {
+      var scope = createScope();
+      var template = '\
+          <md-autocomplete\
+              md-selected-item="selectedItem"\
+              md-search-text="searchText"\
+              md-items="item in match(searchText)"\
+              md-item-text="item.display"\
+              md-min-length="0" \
+              required\
+              placeholder="placeholder">\
+            <span md-highlight-text="searchText">{{item.display}}</span>\
+          </md-autocomplete>';
+
+      var error;
+
+      try {
+        var element = compile(template, scope);
+
+        scope.searchText = undefined;
+        scope.$digest();
+      } catch (e) {
+        error = e;
+      }
+
+      expect(error).toBe(undefined);
+
+      element.remove();
+    });
+  });
+
   describe('md-highlight-text', function() {
     it('should update when content is modified', inject(function() {
       var template = '<div md-highlight-text="query">{{message}}</div>';

src/components/autocomplete/js/autocompleteController.js

@@ -699,7 +699,7 @@ function MdAutocompleteCtrl ($scope, $element, $mdUtil, $mdConstant, $mdTheming,
    * results first, then forwards the process to `fetchResults` if necessary.
    */
   function handleQuery () {
-    var searchText = $scope.searchText,
+    var searchText = $scope.searchText || '',
         term       = searchText.toLowerCase();
     //-- if results are cached, pull in cached results
     if (!$scope.noCache && cache[ term ]) {