hashmap: make sure hashmap_get_strv()+set_get_strv() work with a NULL object

Before we invoke n_entries() we need to check for non-NULL here, like in
all other calls to the helper function. Otherwise we'll crash when
invoked with a NULL object, which we usually consider equivalent to an
empty one though.

src/basic/hashmap.c

@@ -1761,6 +1761,9 @@ char** _hashmap_get_strv(HashmapBase *h) {
         Iterator i;
         unsigned idx, n;
 
+        if (!h)
+                return new0(char*, 1);
+
         sv = new(char*, n_entries(h)+1);
         if (!sv)
                 return NULL;