`fromBundle` doesn't distinguish between `FIELD_BITMAP` and `FIELD_TEXT`
being present with a null value, or being absent, so we might as well
avoid including them when the value is null.
I've separated this from a later change to add
`Cue.toSerializableBundle` which will also skip setting a bitmap value
into the `Bundle` if `this.bitmap == null`. This is partly because it
results in changes to a lot of extractor test dump files, and it's
easier to review that as a separate change.
PiperOrigin-RevId: 586626141
28c210686f