Correct misleading CosmosDB connection string validation error message (#315)

### Description

The current error message when validating `cosmosdb_connection_string`
is misleading.
In the previous version it stated:

> "CosmosDB connection string **CAN BE EMPTY**"

when in fact the correct behavior is that it **cannot** be empty.

This PR updates the `ValueError` to properly reflect the restriction.

### Changes

```python
self._validate_enum_value(kind, CosmosDBVectorSearchType)

if not cosmosdb_connection_string:
    raise ValueError("CosmosDB connection string can't be empty.")

self.cosmosdb_connection_string = cosmosdb_connection_string
self.cosmosdb_client = cosmosdb_client
```

### Expected Behavior

When the `cosmosdb_connection_string` parameter is empty, the following
clear error is raised:

```
ValueError: CosmosDB connection string can't be empty.
```

---------

Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>
Co-authored-by: Mason Daugherty <[email protected]>

Author: 3 people

libs/community/langchain_community/cache.py

@@ -2153,7 +2153,7 @@ def __init__(
         self._validate_enum_value(kind, CosmosDBVectorSearchType)
 
         if not cosmosdb_connection_string:
-            raise ValueError(" CosmosDB connection string can be empty.")
+            raise ValueError("CosmosDB connection string can't be empty.")
 
         self.cosmosdb_connection_string = cosmosdb_connection_string
         self.cosmosdb_client = cosmosdb_client