A "caption" field for images was introduced in #2104 but it seems like the correct backend changes were not introduced. In particular there should have been an exploration migration that checked each image RTE component and added a "caption is empty string" attribute if needed.

The aim of this issue is to write that migration, so that we can ensure that all explorations are up to date and have the correct fields.

/cc @bansalnitish @ishucr7