Don't check height if activity bounds empty

LetterboxUiController#isHorizontalReachabilityEnabled returns false when
ActivityRecord#resolveOverrideConfiguration is called multiple times
during inital loading of apps. This causes brief flicker to center if
letterboxed app position was persisted to left/right since it uses
letterbox horizontal position multiplier instead of correct persister
config.

Fix: 273542824
Test: Manual && atest WmTests:SizeCompatTests
Change-Id: Iaae337bef6ba36b5f3f8cd2edc325d95a2b662fe