hotfix: added backward compatability for advanced params (#29)

* Remove operator and value in payload if they are None in AdvancedParams

Sample:

```python
{
    'app': 'vanilla-search',
    'query': {
        'name': 'query',
        'text': 'himawari',
        'isFirstpage': False,
        'strictRefine': False,
        'removeDuplicates': False,
        'action': 'search',
        'page': 1,
        'pageSize': 10,
        'advanced': {
            'collection': '/user_needs_database/snwg-assessments-2020/',
            # 'value': None, 'operator': None,
        }
    }
}
```

* Bugfix payload generation for AdvancedParams

Now col_name and values are checked to see if empty.

* added support for advanced params according to documentation

* fixed support for single or multiple advanced params

---------

Co-authored-by: NISH1001 <nishanpantha@gmail.com>

pynequa/models.py

@@ -1,4 +1,4 @@
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, Union
 from abc import abstractmethod, ABC
 from dataclasses import dataclass, field
 from loguru import logger
@@ -168,7 +168,7 @@ class QueryParams(AbstractParams):
     aggregations: Optional[List[str]] = field(default_factory=lambda: [])
     order_by: Optional[str] = None
     group_by: Optional[str] = None
-    advanced: Optional[List[AdvancedParams]] = field(
+    advanced: Optional[Union[AdvancedParams, List[AdvancedParams]]] = field(
         default_factory=lambda: [])
     debug: bool = False
 
@@ -256,9 +256,14 @@ def _prepare_query_args(self, query_name: str) -> Dict:
         if self.group_by is not None:
             params["groupBy"] = self.group_by
 
-        if len(self.advanced) > 0:
+        advanced = (
+            [self.advanced]
+            if isinstance(self.advanced, AdvancedParams)
+            else self.advanced
+        )
+        if len(advanced) > 0:
             advanced_param_payload = {}
-            for advanced_param in self.advanced:
+            for advanced_param in advanced:
                 column_name = advanced_param.col_name
                 payload_value = advanced_param.generate_payload()[column_name]
                 if column_name in advanced_param_payload: