Make code for ImageMagick/GraphicsMagick more symmetric

Author: latosha-maltba

Tests/helper.py

@@ -259,11 +259,12 @@ def netpbm_available():
 
 def magick_command():
     if sys.platform == "win32":
-        imagemagick = os.environ.get("MAGICK_HOME", "")
-        if imagemagick:
-            imagemagick = [os.path.join(imagemagick, "convert.exe")]
-            graphicsmagick = [os.path.join(imagemagick, "gm.exe"), "convert"]
+        magickhome = os.environ.get("MAGICK_HOME", "")
+        if magickhome:
+            imagemagick = [os.path.join(magickhome, "convert.exe")]
+            graphicsmagick = [os.path.join(magickhome, "gm.exe"), "convert"]
         else:
+            imagemagick = None
             graphicsmagick = None
     else:
         imagemagick = ["convert"]