basic/hashmap: allow NULL values in strdup hashmaps and add test

src/basic/hashmap.c

@@ -1775,22 +1775,30 @@ int hashmap_put_strdup(Hashmap **h, const char *k, const char *v) {
                 return r;
 
         _cleanup_free_ char *kdup = NULL, *vdup = NULL;
+
         kdup = strdup(k);
-        vdup = strdup(v);
-        if (!kdup || !vdup)
+        if (!kdup)
                 return -ENOMEM;
 
+        if (v) {
+                vdup = strdup(v);
+                if (!vdup)
+                        return -ENOMEM;
+        }
+
         r = hashmap_put(*h, kdup, vdup);
         if (r < 0) {
-                if (r == -EEXIST && streq(v, hashmap_get(*h, kdup)))
+                if (r == -EEXIST && streq_ptr(v, hashmap_get(*h, kdup)))
                         return 0;
                 return r;
         }
 
-        assert(r > 0); /* 0 would mean vdup is already in the hashmap, which cannot be */
-        kdup = vdup = NULL;
+        /* 0 with non-null vdup would mean vdup is already in the hashmap, which cannot be */
+        assert(vdup == NULL || r > 0);
+        if (r > 0)
+                kdup = vdup = NULL;
 
-        return 0;
+        return r;
 }
 
 int set_put_strdup(Set **s, const char *p) {