Fix duplicated variable in baseline noise check

Line 1094 of MassSpectrumClasses.py checked `self.baseline_noise_std`
twice instead of `self.baseline_noise` and `self.baseline_noise_std`.
Also changed from truthiness to `is not None` so valid 0.0 values
are not treated as missing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: robertyoung3

corems/mass_spectrum/factory/MassSpectrumClasses.py

@@ -1091,7 +1091,7 @@ def plot_profile_and_noise_threshold(self, ax=None, legend=False):
         """
         import matplotlib.pyplot as plt
 
-        if self.baseline_noise_std and self.baseline_noise_std:
+        if self.baseline_noise is not None and self.baseline_noise_std is not None:
             # x = (self.mz_exp_profile.min(), self.mz_exp_profile.max())
             baseline = (self.baseline_noise, self.baseline_noise)