refactor: avoid unnecessary alloc by using unwrap_or_else

[etolbakov]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

#3453
#3630

What's changed and what's your intention?

improve the implementation by avoid an allocation if the lookback value is provided.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[etolbakov]

@killme2008 @waynexia @tisonkun
Could you please take a look when you have time?

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.13%. Comparing base (314f270) to head (8eebd91).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3742      +/-   ##
==========================================
- Coverage   85.46%   85.13%   -0.33%     
==========================================
  Files         966      966              
  Lines      162498   162506       +8     
==========================================
- Hits       138871   138348     -523     
- Misses      23627    24158     +531     

[tisonkun]

Thank you!

[tisonkun]

I update the PR title. We're now generating release notes from PR title, so keeping it clear and expressive is important :D

[fengjiachun]

Thank you! LGTM