User::incEditCountImmediate(): Add explicit IS NOT NULL check

User::incEditCountImmediate(): Add explicit IS NOT NULL check

Within the UPDATE query, if user_editcount IS NULL, user_editcount+1 will
also be NULL, and the change becomes a no-op. So in MySQL, the number of
affected rows would be zero, which is the number of rows that actually
changed.

However, other DBMSs (e.g. PostgreSQL, SQLite) do count no-op changes,
meaning the code would not initialize user_editcount if it were NULL.
Explicitly checking for NULL should ensure consistent behavior across
database types.

Also, if the CLIENT_FOUND_ROWS flag is set when connecting to MySQL, the
server returns "Rows matched:" instead of "Changed:" as the affected row
count. This change would be necessary if MediaWiki, like Drupal, is
changed to use that flag <https://www.drupal.org/node/805858>.

Change-Id: Idac160bae56adc5c5d17f8558c55d87000019741