libbinder_ndk: remove logspam on incStrong

AIBinder_incStrong is frequently called on a null binder out of
convenience, because in cases where ownership is required on a binder
that may or may not be nullptr, ignoring the incStrong on nullptr is
consistent.

Bug: 150894288
Test: atest CtsNdkBinderTestCases
Change-Id: I247811a7d4600711c015647f77900bf3183890e2