FIX: Handle zero-valued expression variables properly.

[Samuelopez-ansys]

I noticed the Hfss.evaluate_expression() method was incorrectly returning the symbolic value of an expression, instead of the float value, when the expression evaluated to zero.

For example, this code would print the string 'test - test' instead of 0.0 as expected:

app = Hfss("ZeroVariableTest")
app['x'] = '10 mm'
app["test"] = 'x / 1mm'

print(f'{app.evaluate_expression('test - test')=}') # should be 0.0

I tracked it down to the Variable class constructor, where `si_value' was being directly used as a truth value:

if si_value:
    self._value = si_value
else:
    self._value = self._calculated_value

Unfortunately, float(0.0) is a Falsy value in Python (bool(0.0) == False), so the above incorrectly treats a 0.0 float the same as None. The correct condition for None-ness is if si_value is not None instead of if si_value.

(cherry picked from commit 2e0c753)

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Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.35%. Comparing base (710ce3a) to head (c1d7681).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6376      +/-   ##
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- Coverage   85.36%   85.35%   -0.02%     
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  Files         175      175              
  Lines       65989    65989              
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- Hits        56333    56326       -7     
- Misses       9656     9663       +7     

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