Replace 'Choose...' with blank in select missing words gap selectors

The gap select menus in select missing word qtype may have words/phrases as options in the list. The width of each select menu is based on longest word/phrase within the list, in same group. So, if the words/phrases in a group are too short then the placeholder 'Choose...'  on the top of each list forces the with of the select menu to be the same length as the placeholder. Therefore, the suggestion it to replace the current placeholder with a blank, so that the select menu do not take too much space where options are too short.