to_str() raises TypeError due to float input in np.base_repr()

### Description:
I encountered a TypeError when calling to_str(f: float). The issue occurs because np.base_repr() expects an integer, but the function passes a floating-point number.

### Steps to Reproduce:
```
serializer = NumericSerializer()
print(serializer.to_str(2.5))  # Raises TypeError
```

### Error Message:
TypeError: string indices must be integers
