CLI - pmm-admin summary with empty --filename flag outputs error instead of generating default filename

1. setup pmm-client

2. open CLI and run pmm-admin summary --filename=""

Actual result:

• No such file error received

Expected result:

• .zip file should be created with default (generated) filename like summary_ip{uniqueSuffix}.zip

• user should see summary_ip{uniqueSuffix}.zip created. output