Fix validator to handle null discharge values properly

- Make discharge field optional in data validator
- Remove discharge from required fields list
- Add explicit null check for discharge before float conversion
- Prevent "float() argument must be a string or a real number, not 'NoneType'" errors
- Allow records with valid water levels but malformed/null discharge data

This completes the malformed data handling fix by updating the validator
to match the parser's new behavior of allowing null discharge values.

Before: Validator rejected records with null discharge
After: Validator accepts records with null discharge, validates only if present

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/validators.py

@@ -26,8 +26,8 @@ class DataValidator:
     def validate_measurement(cls, measurement: Dict[str, Any]) -> bool:
         """Validate a single measurement"""
         try:
-            # Check required fields
-            required_fields = ['timestamp', 'station_id', 'water_level', 'discharge']
+            # Check required fields (discharge is now optional)
+            required_fields = ['timestamp', 'station_id', 'water_level']
             for field in required_fields:
                 if field not in measurement:
                     logger.warning(f"Missing required field: {field}")
@@ -38,17 +38,22 @@ class DataValidator:
                 logger.warning(f"Invalid timestamp type: {type(measurement['timestamp'])}")
                 return False
 
-            # Validate water level
+            # Validate water level (required)
+            if measurement['water_level'] is None:
+                logger.warning("Water level cannot be None")
+                return False
             water_level = float(measurement['water_level'])
             if not (cls.WATER_LEVEL_MIN <= water_level <= cls.WATER_LEVEL_MAX):
                 logger.warning(f"Water level out of range: {water_level}")
                 return False
 
-            # Validate discharge
-            discharge = float(measurement['discharge'])
-            if not (cls.DISCHARGE_MIN <= discharge <= cls.DISCHARGE_MAX):
-                logger.warning(f"Discharge out of range: {discharge}")
-                return False
+            # Validate discharge (optional - can be None)
+            discharge_value = measurement.get('discharge')
+            if discharge_value is not None:
+                discharge = float(discharge_value)
+                if not (cls.DISCHARGE_MIN <= discharge <= cls.DISCHARGE_MAX):
+                    logger.warning(f"Discharge out of range: {discharge}")
+                    return False
             
             # Validate discharge percent if present
             if measurement.get('discharge_percent') is not None: