FE: Fix Default Consumer's Lag Value

[K-Diger]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

close #675

The default value of Consumer Lag displayed when searching the Consumer List has been modified.

The existing displayed value is 'N/A', which is different from the response value of '0' that the server responds to.

We believe that 'N/A' may mean a state of disuse, so we write it as 0 to make it clear.

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually (please, describe, if necessary)
• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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