`Invoke-MgGraphRequest` should not require `-Body` to be present to make a `POST` request

[peombwa]

Given https://docs.microsoft.com/en-us/graph/api/user-revokesigninsessions?view=graph-rest-1.0&tabs=http, -Body should be made optional when making POST requests.

Also, Invoke-MgGraphRequest should support relative URIs Irregardless of the -Method.

[georgend]

@psignoret Fixed the relative URL issue and added some tests to confirm it.

I will tackle the -Body issue.