fix(gatsby): account for edge case when payload of DELETE_NODE is undefined

[vladar]

Description

Turns out a payload of the DELETE_NODE action can be undefined in some edge cases (no idea when exactly). And refactor from #27504 didn't account for this. This causes Cannot read property 'id' of undefined' error (again, hard to say in which circumstances).

This PR fixes this problem by using the same strategy as other reducers (just treats this situation as a no-op). Example from other reducer:

gatsby/packages/gatsby/src/redux/reducers/nodes.ts

Lines 21 to 26 in 557139e

	case `DELETE_NODE`: {
	if (action.payload) {
	state.delete(action.payload.id)
	}
	return state
	}