Don't pass NULL to strcmp

A recent gcc update gives me a series of warnings from the NULL
argument to strcmp.  We should be passing an empty string instead
of a NULL pointer.

I suspect this was unintentional from a search/replace operation
in commit 8229e668.

Change-Id: I9677d80d32e5bd54ca1b560788a430c8cf40dae9
Reviewed-on: http://gerrit.openafs.org/1489
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>

src/bucoord/commands.c

@@ -307,7 +307,7 @@ EvalVolumeSet2(struct bc_config *aconfig,
 			    if ((strcmp(&entries[e].name[l], ".backup") == 0)
 				|| (strcmp(&entries[e].name[l], ".readonly")
 				    == 0)
-				|| (strcmp(&entries[e].name[l], NULL) == 0))
+				|| (strcmp(&entries[e].name[l], "") == 0))
 				add = 0;
 			}
 		    }