Log the context of the ULS in which no search results were found

Relies on updates to EventLogging schema. Diff:
https://meta.wikimedia.org/w/index.php?title=Schema%3AUniversalLanguageSelector&type=revision&diff=17799034&oldid=7327441

Bug: T179402
Change-Id: I2ff9ce9b40df16ed3c5e6970fb3b691af08cedec

resources/js/ext.uls.ime.js

@@ -293,6 +293,7 @@
 						$input.focus();
 					},
 					languages: mw.ime.getLanguagesWithIME(),
+					ulsPurpose: 'ime-selector',
 					top: $input.offset().top
 				} );