WebView: defend against null array

This guards against iterating over a null array. This scenario should
never happen on user devices, however we know it can happen if you
install a trunk-staging build of the settings app on top of a nextfood
device configuration. This happens because of a flag mismatch
(updateServiceIpcWrapper is enabled in the trunk-staging app but the
service is not implemented by a nextfood platform build).

This is a low risk change since we know users cannot usually get into
this state.

Fixed: 391725109
Test: N/A - this bug is not possible in any configuration we test
Flag: EXEMPT bugfix
Change-Id: I654e0be5ad94e769cd8325ce5d20d241a4a13c13