Scripting: defaults not initialized in generators

### Bug description

When using a built-in generator from a scripting command, if the command does not explicitly set all parameters, unexpected results occur, which could include crashing Audacity or generating NaNs.

### Steps to reproduce

1. With mod-script-pipe enabled, launch Audacity.
2. Add a track and make a time selection.
3. Send the command "Tone:" to Audacity.

### Expected behavior

A default tone (Frequency=440Hz Amplitude=0.8 Waveform=Sine) to be generated that is the length of the selection.

### Actual behavior

On one occasion, the selection was filled with NaNs.
On another occasion, a tone with a frequency and amplitude that decreased to zero.
On another occasion, Audacity crashed.

### Audacity Version

current master / alpha / nightly build

### Operating system

Linux

### Additional context

Ref: https://manual.audacityteam.org/man/scripting_reference.html

Read the scripting reference for the Tone command, then reproduce the issue with mod-script-pipe using an incomplete command such as "Tone:" and a time selection. Trace the built-in generator path and ensure omitted parameters use the documented defaults: 440Hz, amplitude 0.8, and Sine waveform, producing the selection length without NaNs or a crash.

Written by the indexing model from the issue text.