only pass keyboard to autocomplete when it has selections

otherwise if tab is pressed, try to tab complete the last word

src/components/views/rooms/BasicMessageComposer.js

@@ -265,7 +265,7 @@ export default class BasicMessageEditor extends React.Component {
             handled = true;
         // autocomplete or enter to send below shouldn't have any modifier keys pressed.
         } else if (!(event.metaKey || event.altKey || event.shiftKey)) {
-            if (model.autoComplete) {
+            if (model.autoComplete && model.autoComplete.hasCompletions()) {
                 const autoComplete = model.autoComplete;
                 switch (event.key) {
                     case "ArrowUp":

src/editor/autocomplete.js

@@ -41,6 +41,11 @@ export default class AutocompleteWrapperModel {
         return this._getAutocompleterComponent().hasSelection();
     }
 
+    hasCompletions() {
+        const ac = this._getAutocompleterComponent();
+        return ac && ac.countCompletions() > 0;
+    }
+
     onEnter() {
         this._updateCallback({close: true});
     }