Handle missing documentation gracefully

We need to handle missing documentation by handling nulls. We can't
always extract the documentation from enum entries, those we leave as
optional.

Bug: 442975456
Test: atest frameworks/base/tools/processors/devicepolicy/test/src/android/processor/devicepolicy/test/PolicyProcessorTest.kt
Flag: android.app.admin.flags.policy_streamlining
Change-Id: I39d78914f5a3521b8c9f6c01fea6d80c65229a9c