zfs_get_temporary_prop() should not pass NULL to strcpy()

zfs_get_temporary_prop() should not pass NULL to strcpy()

`dsl_dir_activity_in_progress()` can call `zfs_get_temporary_prop()` with
the forth value set to NULL, which will pass NULL to `strcpy()` when
there is a match

Clang's static analyzer caught this with the help of CodeChecker for
Cross Translation Unit analysis.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14456