Bug report: List-type dashboard variables missing "Set to null" option

Bug Description

Please describe.

On dashboards, list-type SQL variables don't have the "Set to null" toggle that other variable types (string, number, boolean, date) get. So there's no way to actually set a list variable to null from the dashboard UI.

Root cause appears to be in frontend/src/queries/nodes/DataVisualization/Components/Variables/Variables.tsx where the VariableComponent has an early return for list variables when showEditingUI is false (which it always is on dashboards via VariablesForDashboard). This renders a plain LemonSelect dropdown without the LemonSwitch null toggle that the VariableInput popover provides for every other type.

How to reproduce

(Using the reporting customer's queries for the repro examples):

1. Create a SQL insight with a list-type variable (e.g. professional_type with values "student", "professional")
2. Use a coalesce pattern to handle null: AND (coalesce({variables.professional_type}, '') = '' OR professional_type = {variables.professional_type})
3. Add the insight to a dashboard along with insights using other variable types (string, boolean, date)
4. On the dashboard, note that the other variables show a "Set to null" toggle but the list variable only shows its dropdown options, no way to set it to null

Additional context

From: https://posthoghelp.zendesk.com/agent/tickets/49343 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/51283)

Debug info

Debug info
Session: https://us.posthog.com/project/sTMFPsFhdP1Ssg/replay/019c358c-95f2-7ffb-bea9-f55c5b5b44f9?t=0
Admin: http://go/adminOrgEU/018bfcb3-1770-0000-e72a-c6904e30256b (project ID 122423)

Start in frontend/src/queries/nodes/DataVisualization/Components/Variables/Variables.tsx, focusing on VariableComponent and the dashboard path through VariablesForDashboard. Compare the list-variable rendering with the VariableInput popover and verify that dashboard list variables expose the existing "Set to null" control. Done means a list variable can be set to null from the dashboard UI like the other variable types.