[sw, dif_uart] Add null checks to send and receive routines
[sw, dif_uart] Add null checks to send and receive routines

These parameters must be valid, and hence must be checked.

Signed-off-by: Silvestrs Timofejevs <5a6466a3751cccdb6ee413d1e87698ed8baaba81@lowrisc.org>

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