Checks viewId's nullibility before findA11yNodeInfos

Returns empty list for AccessibilityNodeInfo#findAccessibilityNodeInfosByViewId
if the viewId is null to prevent NullPointerException happened to the UI client.

Bug: 158357326
Test: manually test
Change-Id: I041d379ed2866ac9d67067b1fcde4c5b585a95ad
3 files changed