TPT 1880: Fixed issue with updating non-populated NodeBalancerNode (#277)

## 📝 Description

Previously, attempting to update a non-populated NodeBalancerNode would
result in the update actually updating the resource silently. This
change fixes the issue.

## ✔️ How to Test

`pytest test`

Note: Since this change deals with making updates to real resources as
opposed to fixtures, it cannot be tested using mocks and must therefore
be tested manually. To do this, first create a NodeBalancerNode in your
Linode account if one does not already exist, and then run this python
script and verify that the weight of the node was actually updated.

```
#!/usr/bin/env python3

from linode_api4 import LinodeClient
from linode_api4.objects import  NodeBalancerNode

client = LinodeClient(<personal_access_token>)

node = NodeBalancerNode(client, <node_id>, <config_id>, <nodebalancer_id>)

node.weight = 60

node.save()
```

Resolves #97

Author: ezilber-akamai-zz

linode_api4/objects/base.py

@@ -210,9 +210,26 @@ def save(self, force=True) -> bool:
         if not force and not self._changed:
             return False
 
-        resp = self._client.put(
-            type(self).api_endpoint, model=self, data=self._serialize()
-        )
+        data = None
+        if not self._populated:
+            data = {
+                a: object.__getattribute__(self, a)
+                for a in type(self).properties
+                if type(self).properties[a].mutable
+                and object.__getattribute__(self, a) is not None
+            }
+
+            for key, value in data.items():
+                if (
+                    isinstance(value, ExplicitNullValue)
+                    or value == ExplicitNullValue
+                ):
+                    data[key] = None
+
+        else:
+            data = self._serialize()
+
+        resp = self._client.put(type(self).api_endpoint, model=self, data=data)
 
         if "error" in resp:
             return False