fix get group members ids to ignore empty start

[twlin0802]

Issue: #474

[CLAassistant]

All committers have signed the CLA.

[habara-k]

Thank you for always using line-sdk-go and sharing the issue.

This code is automatically generated according to the open API specifications, so we cannot address this by editing it directly.

There are other issues in the Go SDK where null cannot be specified:

• Empty contents are encoded to null #468
• Is limit.max optional? #466
• How can I call IssueStatelessChannelToken in the case of without client_assertion and client_assertion_type #464

We are currently working on fixing this type of bug by modifying https://github.com/line/line-bot-sdk-go/tree/master/generator/src/main/java/line/bot/generator, but we do not have the resources to allocate to this task at the moment.

We apologize for the inconvenience and appreciate your patience.

[twlin0802]

Got it, then I'll close this PR. Thanks for your reply.