Fix last_process related bug in RPC.health

tests/rpc/test_rpc.py

@@ -1252,6 +1252,6 @@ def test_rpc_health(mocker, default_conf) -> None:
 
     freqtradebot = get_patched_freqtradebot(mocker, default_conf)
     rpc = RPC(freqtradebot)
-    result = rpc._health()
-    assert result['last_process'] == '1970-01-01 00:00:00+00:00'
-    assert result['last_process_ts'] == 0
+    result = rpc.health()
+    assert result['last_process'] is None
+    assert result['last_process_ts'] is None

tests/rpc/test_rpc_apiserver.py

@@ -1801,8 +1801,8 @@ def test_health(botclient):
 
     assert_response(rc)
     ret = rc.json()
-    assert ret['last_process_ts'] == 0
-    assert ret['last_process'] == '1970-01-01T00:00:00+00:00'
+    assert ret["last_process_ts"] is None
+    assert ret["last_process"] is None
 
 
 def test_api_ws_subscribe(botclient, mocker):