Fix crash when potential null authority passed to build rootsUri

The Uri class has the getAuthority method as @Nullable. Unfortunately in
the isDocumentsProvider method it doesn't check if the authority String
is null and this can lead to a null dereference when attempting to call
.equals(). This updates the annotation appropriately and bails early if
the string is null.

Bug: 412896040
Test: atest testRootsUri_returnsFalse
Flag: EXEMPT bug fix
Change-Id: I150de4d07889b714137c9c1bfe8a05458ff477a3