Remove syntax warning

[max-sixty]

Currently raises a SyntaxWarning (though I don't think causing any issues)

/home/vsts/work/1/s/xarray/plot/plot.py:291: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if args is ():

[crusaderky]

Yes, is () instead of == () accidentally works on CPython because in order to save RAM CPython has a bunch of hardcoded objects that are defined at interpreter level. That's however an implementation detail, and there's no guarantee it won't break on an alternative implementation, or even on a future version of CPython.

>>> a = ()                                                                 
>>> b = ()                                                                 
>>> a is b                                                                 
True
>>> a = (1, 2)                                                             
>>> b = (1, 2)                                                             
>>> a is b                                                                 
False
>>> x = 1                                                                  
>>> y = 1                                                                  
>>> x is y                                                                 
True
>>> x = 1000                                                          
>>> y = 1000                                                      
>>> x is y                                                                 
False
>>> for x in range(1000): 
...     y = int(str((x))) 
...     if x is not y: 
...         print(x) 
...         break                                                                         
257