Refresh cagg uses min value for dimension when start_time is NULL

[gayyappan]

When the refresh_continuous_aggregate window's start is NULL, use the min value in the hypertable to determine the beginning of the range instead of the min value for the partition column.

[codecov]

Codecov Report

Attention: Patch coverage is 10.81081% with 33 lines in your changes missing coverage. Please review.

Project coverage is 82.15%. Comparing base (59f50f2) to head (8c32688).
Report is 660 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hypertable.c	0.00%	25 Missing ⚠️
tsl/src/continuous_aggs/refresh.c	33.33%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7546      +/-   ##
==========================================
+ Coverage   80.06%   82.15%   +2.09%     
==========================================
  Files         190      230      +40     
  Lines       37181    43360    +6179     
  Branches     9450    10912    +1462     
==========================================
+ Hits        29770    35624    +5854     
- Misses       2997     3412     +415     
+ Partials     4414     4324      -90     

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