Add null checks in CursorWindow#put{String,Blob}.

Currently passing null values to putString or putBlob will
crash in native code, which is not a great developer experience.

This commit adds null checks so the java layer will throw NPEs.

Flag: EXEMPT bugfix
Test: atest android.database.CursorWindow
Bug: 351839899
Change-Id: Ic4f1c55d0edb56fa08fabf4be6193afb6e86c3e6