fix(sync): check the start block request height

[b00f]

Description

This PR checks if the start block request height is available; otherwise, it rejects the download request.

[codecov]

Codecov Report

Merging #1176 (51f42e5) into main (885d046) will increase coverage by 0.08%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 51f42e5 differs from pull request most recent head 3e197fa. Consider uploading reports for the commit 3e197fa to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1176      +/-   ##
==========================================
+ Coverage   80.64%   80.73%   +0.08%     
==========================================
  Files         178      178              
  Lines        9452     9457       +5     
==========================================
+ Hits         7623     7635      +12     
+ Misses       1453     1446       -7     
  Partials      376      376