Add null check for request.json in market_holidays, symbol, and intervals endpoints

[marketcalls]

Description

Several API endpoints pass `request.json` directly to schema validation without checking if it's `None`. If a client sends a request with no JSON body or incorrect Content-Type, this results in an unhandled error instead of a clean error response.

Affected Files

• `restx_api/market_holidays.py` (line 30)
• `restx_api/symbol.py` (line 30)
• `restx_api/intervals.py` (line 31)

Reference Implementation

`restx_api/option_greeks.py` (line 84) already handles this correctly:

```python
if request.json is None:
return make_response(jsonify({
"status": "error",
"message": "Request body must be JSON"
}), 400)

data = schema.load(request.json)
```

What to Do

1. Add the null check before schema validation in each affected file
2. Follow the pattern from `option_greeks.py`

Acceptance Criteria

• All three files validate `request.json` is not None
• Returns 400 with clear error message for non-JSON requests
• Existing valid requests still work

Skills You'll Learn

• API input validation
• Defensive programming
• Flask request handling

[LuckyAnsari22]

I'll take this. The pattern from option_greeks.py is clean — I'll replicate it across all three files and make sure the error message format stays consistent.