Make medium titles VARCHAR NOT NULL

The titles for individual mediums of a release are defined as VARCHAR(255) and can be NULL. This is inconsistent with other cases of optional string values, where the empty string is used to indicate the absence of a value (e.g., disambiguation comment on all types of entities, address for places); there is also no semantic distinction that could be expressed by a NULL value as opposed to an empty string. Further, this is inconsistent with other entity names, which are VARCHAR without a length limit.

Therefore,

• all existing NULL medium titles should be updated to the empty string,
• the definition for medium.name should be changed to VARCHAR NOT NULL DEFAULT ''.

Currently, there are 1522931 mediums with a NULL title and 3224 mediums with a title that is the empty string; this is because those 3224 mediums were added by Likedis Auto.