remoteproc: Don't handle empty resource table

Allow a NULL table_ptr to have the same meaning as a table with 0
entries, allowing a subsequent patch to skip the assignment step.

A few other places in the implementation does dereference table_ptr, but
they are currently all coming from rproc_handle_resources().

Reviewed-By: Loic Pallardy <loic.pallardy@st.com>
Tested-By: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>