grade/report/overview/index.php and grade/report/user/index.php call the same single_select.

grade/report/overview/index.php:
"Choose..." displays empty main content. I think from looking at the code it is supposed to select the current user but it's not working.

grade/report/user/index.php:
no need "Choose..." here. I think the correct solution is to remove the "Choose...", there is some code for grade report overview for "Choose..." but it makes no sense to me.

I think we can remove the "Choose..." option. A fix get be extracted from https://github.com/mouneyrac/moodle/compare/c36a240...MDL-41075-master.

Test instructions: go to grade/report/overview/index.php?id=COURSEID and grade/report/user/index.php?id=COURSEID, check the user selector doesn't have "Choose..."

Update:
Marina detected an error for this fix: https://tracker.moodle.org/browse/MDL-41075?focusedCommentId=261299&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-261299

your change in overview report is actually creating a bug. I logged in as admin user, without "Choose" in dropdown the first "Student1" is selected which means that I have no possibility to view the Student1's report.

So removing "Choose..." may not be the best solution.