FE: Do not show messages behind = 0, when it is not set on backend

Issue submitter TODO list

• I've looked up my issue in FAQ
• I've searched for an already existing issues here
• I've tried running master-labeled docker image and the issue still persists there
• I'm running a supported version of the application which is listed here

Describe the bug (actual behavior)

When on Consumers page if consumer has no committed offsets, now it is show like message behind = "0", which is not right.

Expected behavior

No value should be shown. (of disclaimer that "No offsets have been committed yet")

Your installation details

0.7.1

Steps to reproduce

1. start consumer that reads any topic
2. go to Consumers -> select consumer -> see messages behind column (on topic level)

Screenshots

Logs

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Additional context

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