[fix #3087] Allow NULL for #__content columns

+ Allow null value for, `#__content` table `images`, `urls`, `attribs` and `metadata` columns.
+ Please follow testing guidelines from here https://github.com/joomla/joomla-cms/issues/3087
should be
ALTER TABLE #__content ALTER COLUMN images SET DEFAULT '{}';
ALTER TABLE #__content ALTER COLUMN urls SET DEFAULT '{}';
ALTER TABLE #__content ALTER COLUMN attribs SET DEFAULT '{}';
ALTER TABLE #__content ALTER COLUMN metadata SET DEFAULT '{}';