Fixes #6765 - correct wrong validation on media name

Regex used for validation was wrong, raising error on multiple white spaces instead of on trailing white space.
Trailing white space is stripped anyways by StripWhitespace concern before validation.
Original bug was concerning " " passing validation. However, that is not a single white space but rather quotes with a space between them; while not useful, it is a valid name. (The quotes are escaped in the db)