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/ether-addr-util.c

@@ -71,7 +71,7 @@ int ether_addr_from_string(const char *s, struct ether_addr *ret, size_t *offset
                         if (s[pos] == '\0')                     \
                                 break;                          \
                         hexoff = strchr(hex, s[pos]);           \
-                        if (hexoff == NULL)                     \
+                        if (!hexoff)                            \
                                 break;                          \
                         assert(hexoff >= hex);                  \
                         x = hexoff - hex;                       \
@@ -99,7 +99,7 @@ int ether_addr_from_string(const char *s, struct ether_addr *ret, size_t *offset
         sep = s[strspn(s, hex)];
         if (sep == '\n')
                 return -EINVAL;
-        if (strchr(":.-", sep) == NULL)
+        if (!strchr(":.-", sep))
                 return -EINVAL;
 
         if (sep == '.') {