Don't fail to deserialize a ZenRule if it has an invalid filter value

This was the only case where readRuleXml() returned null. Making it NonNull simplifies callers (and removes a possible exception when the manual rule is involved).

Fixes: 357902620
Test: atest ZenModeConfigTest ZenModeHelperTest
Flag: EXEMPT trivial fix
Change-Id: Ibe1884e6b286d32f845de143ef526ac3ec38fa8f