local_assigns already returns nil for missing keys

app/views/shared/issuable/_filter.html.haml

 - finder = controller.controller_name == 'issues' || controller.controller_name == 'boards' ? issues_finder : merge_requests_finder
 - boards_page = controller.controller_name == 'boards'
-- board = local_assigns.fetch(:board, nil)
+- board = local_assigns[:board]
 
 .issues-filters
   .issues-details-filters.row-content-block.second-block

app/views/shared/issuable/_milestone_dropdown.html.haml

 - project = @target_project || @project
 - extra_class = extra_class || ''
 - show_menu_above = show_menu_above || false
-- board = local_assigns.fetch(:board, nil)
-- board_has_milestone = board.try(:milestone).present?
-- selected = local_assigns.fetch(:selected, nil)
-
-- if board_has_milestone
-  - selected = board.milestone
+- board = local_assigns[:board]
+- selected = board.try(:milestone) || local_assigns[:selected]
 
 - selected_text = selected.try(:title) || params[:milestone_title]
 - dropdown_title = local_assigns.fetch(:dropdown_title, "Filter by milestone")