Fix addViewAt crash on a ViewState with no view

[wneel]

Summary

Fixes #58526.

SurfaceMountingManager.addViewAt calls checkNotNull(view) on a ViewState whose view can
legitimately be null, which throws and tears the surface down. A ViewState has no Android View in
two cases: createViewUnsafe only creates one when the node is layoutable, and updateEventEmitter
registers a bare placeholder via getOrPut for a tag it has never seen.

In production (#58526) it is the second case. The reported string
ViewState [3548] - isRoot: false - props: null - viewManager: null has props: null, which only
the updateEventEmitter placeholder can produce, since createViewUnsafe always assigns
currentProps before registering, and the root ViewState prints isRoot: true.

Either way the mount instruction stream is inconsistent, and this function already has an
established answer for that: the branch immediately above logs a soft exception and returns when the
ViewState is missing entirely. This makes the null-view case behave the same way, so a missing
Create degrades to a dropped insert rather than a dead surface. Same approach as #56389 and #57181
took for their branches.

Changelog:

[ANDROID] [FIXED] - Stop SurfaceMountingManager.addViewAt from crashing on a ViewState with no view

Test Plan

Added addViewAt_doesNotThrowWhenViewStateHasNoView to the existing
SurfaceMountingManagerTest. It preallocates a tag with isLayoutable = false, so a ViewState is
registered with no Android View, then calls addViewAt on it. Fails on main with
IllegalStateException: Unable to find view for viewState ... and tag 42, passes with this change.

No native code and no EventEmitterWrapper needed, so it runs under Robolectric like the rest of
the file. The neighbouring deleteView_isIdempotent test already relies on a soft exception not
throwing in this setup.