Disallow to have empty billing plans list

[m-mohr]

Discussed in #420

^{Originally posted by soxofaan October 28, 2021}

openeo-api/openapi.yaml

Lines 746 to 750 in 592e266

	plans:
	description: Array of plans
	type: array
	items:
	title: Billing Plan

just checking: is it intentional that the billing plans array can be empty?

This is not intentional and we should set minItems to 0 once we are going for 2.0.