Number Input fails on float conversion

[bergmansj]

Description

Using the basic number input example from the docs:

from nicegui import ui

ui.number(label='Number', value=3.1415927, format='%.2f',
          on_change=lambda e: result.set_text(f'you entered: {e.value}'))
result = ui.label()

ui.run()

When you empy out the input field (or even use backspace to make '3.' from '3.14') the following error appears on the console:

could not convert string to float: ''
Traceback (most recent call last):
  File " <snip> /venv/lib/python3.10/site-packages/nicegui/events.py", line 373, in handle_event
    result = handler(arguments) if expects_arguments else handler()
  File " <snip> /venv/lib/python3.10/site-packages/nicegui/elements/mixins/value_element.py", line 30, in handle_change
    self.set_value(self._event_args_to_value(e))
  File "  <snip>  /venv/lib/python3.10/site-packages/nicegui/elements/number.py", line 97, in _event_args_to_value
    return float(e.args)
ValueError: could not convert string to float: ''

(local paths snipped)

This was introduced somewhere after v1.3.6 (that version has a different _event_args_to_value implementation that checks for an empty string)

[falkoschindler]

Wow, thanks for reporting this bug, @bergmansj!

Minimum reproduction:

ui.number()

→ Type "1" followed by backspace.

[falkoschindler]

Here I introduced the bug while "fixing some mypy issues":

a55a452#diff-876b4c6ca305085724524c58b88f3685057fb0a803275d6957375205a96dfb17

When removing all characters from the number input, the value becomes '' instead of None.
I fixed the condition in a001cab.