Fix to_rgba to recognise an empty string

Author: has2k1

mizani/_colors/utils.py

@@ -103,7 +103,7 @@ def to_rgba(
         RGBA color(s)
     """
     if isinstance(colors, str):
-        if colors == "none" or colors == "None":
+        if colors in ("none", "None", ""):
             return "none"
 
         if isinstance(alpha, (float, int, np.floating, np.integer)):

tests/colors/test_utils.py

@@ -31,6 +31,8 @@ def test_to_rgba():
     assert to_rgba("red", np.int64(1)) == "#FF0000FF"  # pyright: ignore[reportCallIssue,reportArgumentType]
 
     assert to_rgba(None, 1) == "none"
+    assert to_rgba("", 1) == "none"
+    assert to_rgba("none", 0.2) == "none"
     assert to_rgba([None, None], 1) == ["none", "none"]  # pyright: ignore[reportCallIssue,reportArgumentType]
 
     with pytest.raises(ValueError):