Disallow passing arguments like -XNone to GMT (#639)

Arguments like -XNone are invalid for GMT.

This PR updates the function build_arg_string and checks if any value is None.
If yes, remove the argument.

pygmt/helpers/utils.py

@@ -127,7 +127,11 @@ def build_arg_string(kwargs):
     Examples
     --------
 
-    >>> print(build_arg_string(dict(R="1/2/3/4", J="X4i", P="", E=200)))
+    >>> print(
+    ...     build_arg_string(
+    ...         dict(R="1/2/3/4", J="X4i", P="", E=200, X=None, Y=None)
+    ...     )
+    ... )
     -E200 -JX4i -P -R1/2/3/4
     >>> print(
     ...     build_arg_string(
@@ -147,6 +151,8 @@ def build_arg_string(kwargs):
         if is_nonstr_iter(kwargs[key]):
             for value in kwargs[key]:
                 sorted_args.append("-{}{}".format(key, value))
+        elif kwargs[key] is None:  # arguments like -XNone are invalid
+            continue
         else:
             sorted_args.append("-{}{}".format(key, kwargs[key]))