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();
        }
    }