Fix the check logic

Author: seisman

pygmt/params/box.py

@@ -86,9 +86,10 @@ def validate(self):
         Validate the parameters.
         """
         # shading_offset must be a sequence of two values or None.
-        if (
-            self.shading_offset and not isinstance(self.shading_offset, Sequence)
-        ) or len(self.shading_offset) != 2:
+        if self.shading_offset and (
+            not isinstance(self.shading_offset, Sequence)
+            or len(self.shading_offset) != 2
+        ):
             raise GMTValueError(
                 self.shading_offset,
                 description="value for parameter 'shading_offset'",