Repair StandardLoad parsing from .mat/.raw files

[HaleyRoss]

Removed if statement from the read_loads!() function that caused StandardLoads being read from .mat and .raw files to be dropped if the constant_active_power and constant_reactive_power arguments were 0.

This is correcting issue #1032

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.25%. Comparing base (4587ed0) to head (a982911).

:exclamation: Current head a982911 differs from pull request most recent head 4999e39. Consider uploading reports for the commit 4999e39 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1065      +/-   ##
==========================================
+ Coverage   85.18%   85.25%   +0.06%     
==========================================
  Files         170      170              
  Lines        7702     7695       -7     
==========================================
- Hits         6561     6560       -1     
+ Misses       1141     1135       -6     

Flag	Coverage Δ
unittests	85.25% <100.00%> (+0.06%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/parsers/power_models_data.jl	91.04% <100.00%> (-0.03%)	:arrow_down:

... and 1 file with indirect coverage changes

[jd-lara]

@HaleyRoss can you add a test for this please?