lindebugfs: check that name is set as otherwise pfs_alloc_node_flags() panics

I have hit the case multiple times that some LinuxKPI field may not be
set during driver bringup and lindebugfs would cause a panic.
The backtrace goes like:

strlen() at strlen+0x54
pfs_create_dir() at pfs_create_dir+0x41
debugfs_create_dir() at debugfs_create_dir+0xa1
...

While the problem is clearly in LinuxKPI or the driver, we likely
should at least add an assert to pfs_create_dir() if name is NULL
like we have for pfs_add_node() but for lindebugfs at least make
this a graceful error and continue without creating the dir instead
of panicing.

Sponsored by: The FreeBSD Foundation
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D54944

(cherry picked from commit 4b32fdb4a614de602d288ac5a414cb43e9f2163e)