[fdio] Rationalize default handling for null read/write

Previously, the default behavior for read was to succeed at reading zero
bytes and the default behavior for write was to succeed at writing all
the bytes (even though nothing actually happened in either case).

After this CL, these operations fail by default and the exact failure
behavior is consistent between fdio and zxio.

Test: null-test
Change-Id: I22587be52de3571ccefd25f21ec20180b3380cf1