page_size required in cli subscriptions list, not defaulting

[charcey]

Expected behavior
page_size should not be required, per docs when using planet subscriptions list

Actual behavior (describe the problem)
planet subscriptions list yields Error: {"error":{"reason":"Request failed schema validation","details":["parameter \"page_size\" in query has an error: empty value is not allowed"]}}

This is a breaking change for users using this command who don't want to specify a page size and continue to use the default.

Related Issues
#1096

Workaround
Always specify --page_size=number in subscriptions list commands