tree-wide: drop a few == NULL and != NULL comparison

Our CODING_STYLE suggests not comparing with NULL, but relying on C's
downgrade-to-bool feature for that. Fix up some code to match these
guidelines. (This is not comprehensive, the coccinelle output for this
is unfortunately kinda borked)

src/basic/path-util.c

@@ -538,7 +538,7 @@ bool paths_check_timestamp(const char* const* paths, usec_t *timestamp, bool upd
 
         assert(timestamp);
 
-        if (paths == NULL)
+        if (!paths)
                 return false;
 
         STRV_FOREACH(i, paths) {