GridState breaks when clearing an advanced filter

[emteknetnz]

Affects 4.x, though not 4.7.x-dev

I think this is a regression caused by #9190

I think it's responsible for this failure (though I'm not 100% certain)
https://travis-ci.com/github/silverstripe/silverstripe-admin/jobs/470323964

silverstripe-admin behat:
manage-users.feature:59

Replication steps:

• Go to the security section
• Create a user 'myuser@example.com'
• Click the advanced filter
• Type in 'myuser' in the email field and filter
• Clear the filter
• The gridfield has not reset - the Default admin user is not showing

If you view html source of the GridState:

4.x-dev

<input type="hidden" name="Members[GridState]" value="{}" class="gridstate" id="Members_GridState" />

4.7.x-dev

<input type="hidden" name="Members[GridState]" value="{&quot;GridFieldSortableHeader&quot;:{&quot;SortColumn&quot;:[]},&quot;GridFieldFilterHeader&quot;:{&quot;Columns&quot;:null},&quot;GridFieldPaginator&quot;:{&quot;currentPage&quot;:1,&quot;itemsPerPage&quot;:15}}" class="gridstate" id="Members_GridState" />

@maxime-rainville

Pull requests

• BUG Reset GridFieldFilterHeader grid state when search is cleared #9829