Check for insertion in ChatModifier

[MasterDash5]

Mojang forgot to check if a component contains an insertion when serializing to JSON. This means if a component's style only contains an insertion, the serializer won't add the insertion to the JSON.

Insertion is missing:
/tellraw @a {"text":"test","insertion":"test"}

Insertion is present:
/tellraw @a {"text":"test","insertion":"test","color":"white"}

This PR addds a check for insertion in ChatModifier.g(), which is used to check if a component contains any styling.
PandaAdventure also uses this serializer for NMS <-> Adventure, so the current Adventure implementation is also affected by this.