Use 0 instead of NULL for comparison to char16_t

musl libc defines NULL as nullptr, which is explicitly allowed by
C++11.  nullptr_t cannot be implicitly cast to an integral type.
Use 0 instead.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: I0c3b6c94cd69262f574414bf52494333f2f2645a