Fix dotplot vcenter=0 being ignored due to falsy check

`if vcenter:` evaluates to False when vcenter=0, so TwoSlopeNorm was
never applied. Changed to `if vcenter is not None:` and added vcenter=0
to the parametrized test.

Closes #293

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

Author: PauBadiaM

src/decoupler/pl/_dotplot.py

@@ -102,7 +102,7 @@ def dotplot(
     # Plot
     ns = (s_vals * scale * plt.rcParams["lines.markersize"]) ** 2
     bp.ax.grid(axis="x")
-    if vcenter:
+    if vcenter is not None:
         norm = TwoSlopeNorm(vmin=None, vcenter=vcenter, vmax=None)
     else:
         norm = None

tests/pl/test_dotplot.py

@@ -20,7 +20,7 @@ def df():
     return df
 
 
-@pytest.mark.parametrize("vcenter", [None, 3])
+@pytest.mark.parametrize("vcenter", [None, 0, 3])
 def test_dotplot(
     df,
     vcenter,