Fix NULL pointer passed to strlcpy from zap_lookup_impl()

Clang's static analyzer pointed out that whenever zap_lookup_by_dnode()
is called, we have the following stack where strlcpy() is passed a NULL
pointer for realname from zap_lookup_by_dnode():

strlcpy()
zap_lookup_impl()
zap_lookup_norm_by_dnode()
zap_lookup_by_dnode()

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14044