Python: Fix missed assertion bug when loading from .env file (#2274)

### Motivation and Context

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When loading config from .env, the assertions like `assert api_key is
not None, "Azure OpenAI API key not found in .env file"` don't catch
anything. While `dotenv_values(".env")` does put None values for keys
with missing values, that's only if there's no equals sign after the
key. For example, this file:
```
OPENAI_API_KEY=""
OPENAI_ORG_ID="org_id"
AZURE_OPENAI_DEPLOYMENT_NAME=
AZURE_OPENAI_ENDPOINT
```
Would be parsed like this:
```
OrderedDict([('OPENAI_API_KEY', ''), ('OPENAI_ORG_ID', 'org_id'), ('AZURE_OPENAI_DEPLOYMENT_NAME', ''), ('AZURE_OPENAI_ENDPOINT', None)])
```

As you can see, OPENAI_API_KEY and AZURE_OPENAI_DEPLOYMENT_NAME are
loaded with empty strings, not None.
### Description

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These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

---------

Co-authored-by: Shawn Callegari <36091529+shawncal@users.noreply.github.com>

python/semantic_kernel/utils/settings.py

@@ -17,7 +17,7 @@ def openai_settings_from_dot_env() -> Tuple[str, Optional[str]]:
     api_key = config.get("OPENAI_API_KEY", None)
     org_id = config.get("OPENAI_ORG_ID", None)
 
-    assert api_key is not None, "OpenAI API key not found in .env file"
+    assert api_key, "OpenAI API key not found in .env file"
 
     # It's okay if the org ID is not found (not required)
     return api_key, org_id
@@ -40,12 +40,10 @@ def azure_openai_settings_from_dot_env(include_deployment=True) -> Tuple[str, st
 
     # Azure requires the deployment name, the API key and the endpoint URL.
     if include_deployment:
-        assert (
-            deployment is not None
-        ), "Azure OpenAI deployment name not found in .env file"
+        assert deployment, "Azure OpenAI deployment name not found in .env file"
 
-    assert api_key is not None, "Azure OpenAI API key not found in .env file"
-    assert endpoint is not None, "Azure OpenAI endpoint not found in .env file"
+    assert api_key, "Azure OpenAI API key not found in .env file"
+    assert endpoint, "Azure OpenAI endpoint not found in .env file"
 
     return deployment or "", api_key, endpoint
 
@@ -60,9 +58,7 @@ def postgres_settings_from_dot_env() -> str:
     config = dotenv_values(".env")
     connection_string = config.get("POSTGRES_CONNECTION_STRING", None)
 
-    assert (
-        connection_string is not None
-    ), "Postgres connection string not found in .env file"
+    assert connection_string, "Postgres connection string not found in .env file"
 
     return connection_string
 
@@ -79,7 +75,7 @@ def pinecone_settings_from_dot_env() -> Tuple[str, str]:
     api_key = config.get("PINECONE_API_KEY", None)
     environment = config.get("PINECONE_ENVIRONMENT", None)
 
-    assert api_key is not None, "Pinecone API key not found in .env file"
-    assert environment is not None, "Pinecone environment not found in .env file"
+    assert api_key, "Pinecone API key not found in .env file"
+    assert environment, "Pinecone environment not found in .env file"
 
     return api_key, environment