fix: allow up/down normally for no completions

Autocomplete current eats up up/down key events by unconditionally returning
true for onUpArrow and onDownArrow. Instead, only do that if there are completions actually visible.

src/components/views/rooms/Autocomplete.js

@@ -64,6 +64,9 @@ export default class Autocomplete extends React.Component {
     onUpArrow(): boolean {
         let completionCount = this.countCompletions(),
             selectionOffset = (completionCount + this.state.selectionOffset - 1) % completionCount;
+        if (!completionCount) {
+            return false;
+        }
         this.setSelection(selectionOffset);
         return true;
     }
@@ -72,6 +75,9 @@ export default class Autocomplete extends React.Component {
     onDownArrow(): boolean {
         let completionCount = this.countCompletions(),
             selectionOffset = (this.state.selectionOffset + 1) % completionCount;
+        if (!completionCount) {
+            return false;
+        }
         this.setSelection(selectionOffset);
         return true;
     }

src/components/views/rooms/MessageComposerInput.js

@@ -503,18 +503,14 @@ export default class MessageComposerInput extends React.Component {
     }
 
     onUpArrow(e) {
-        if(this.props.onUpArrow) {
-            if(this.props.onUpArrow()) {
-                e.preventDefault();
-            }
+        if (this.props.onUpArrow && this.props.onUpArrow()) {
+            e.preventDefault();
         }
     }
 
     onDownArrow(e) {
-        if(this.props.onDownArrow) {
-            if(this.props.onDownArrow()) {
-                e.preventDefault();
-            }
+        if (this.props.onDownArrow && this.props.onDownArrow()) {
+            e.preventDefault();
         }
     }