Updates the realm field `default_code_block_language` to have a default
value of an empty string instead of None. Also updates the web-app to
check for the empty string and not `null` to indicate no default is set.
This means that both new realms and existing realms that have no default
set will have the same value for this setting: an empty string.
Previously, new realms would have None if no default was set, while realms
that had set and then unset a value for this field would have an empty
string when no default was set.
