LibSQL: Relax assignment rules for Null Values

It should be possible to assign a Value of any type to a Value which
currently is Null.

Userland/Libraries/LibSQL/Value.cpp

@@ -261,6 +261,8 @@ Value& Value::operator=(Value const& other)
     if (this != &other) {
         if (other.is_null()) {
             assign(null());
+        } else if (is_null()) {
+            assign(other);
         } else {
             VERIFY(can_cast(other));
             assign(other);