Omit empty cursor parameter in Compute ACL's ListEntries requests

[philippschulte]

Change summary

This PR fixes an issue where the go-fastly client would send a ?cursor= query parameter in ListEntries requests when no cursor value was provided by the user.

Recent upstream API behavior now rejects this with a 400 Bad Request: invalid cursor error. Previously, this was either tolerated by the API or untested — but we cannot definitively prove whether the behavior changed or was always like this.

Root cause

• The CLI (which uses go-fastly) sets Cursor = "" by default when the --cursor flag is not provided, due to Go’s default zero value for strings.
• In go-fastly, the ListEntries function would include all non-nil values in the query parameters — so cursor="" resulted in a request like:

  GET /resources/acls/<id>/entries?cursor=
  

• This caused the API to return a 400 error, even though cursor is documented as optional.

Fix

We now explicitly omit the cursor parameter from the request if:

• it is nil, or
• it is an empty string ("")

This matches the intended behavior and prevents accidental inclusion of invalid cursor values.

Test coverage

• A regression test has been added to the main integration test (TestClient_ComputeACL) to verify that calling ListEntries with cursor="" does not result in an error.
• Previously, we had no test case that passed an explicit empty cursor, which allowed this issue to go undetected.

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