NumberInput Widget does not work if instantiated with non-int step

### Describe the bug

When a `toga.widgets.NumberInput` is instantiated and a step is given that is `float`, `str`, or `decimal.Decimal` the value property will always return `None`.

Tested on Linux Mint, Python 3.12.3, toga 0.5.3

### How to reproduce the bug

1. run example code with toga 0.5.3 installed
2. asserts will fail

### Minimum example code

```shell
import toga

number_input = toga.NumberInput(step=1.0, value=1)
assert number_input.value is not None
number_input.value = 2
assert number_input.value is not None
```

### Screenshots

_No response_

### Environment details

- Operating system and version: Linux Mint 22.3 - Cinnamon 64-bit
- Python version: 3.12.3
- Software versions:
- Toga: 0.5.3

### Logs

```

```

### Additional context

_No response_

Start by reproducing the example with toga.NumberInput using float, str, and decimal.Decimal step values, then locate the NumberInput implementation and its existing tests. Done means the value property is non-None both after construction and after assignment for these step types.

Written by the indexing model from the issue text.