Remove redundant test
compute_null_map() for element-less FE reinit

This avoids a segfault in the case where an FE user does an
element-less reinit() but inadvertently leaves calculate_dphi or
calculate_d2phi on.
compute_map even in element-less FE::reinit

It will be a dull map in this case, but this way we keep all our data
vectors the same size, which assumption is scattered through other FE
code.