Update graphics.py (#127)

Generally, the variable "bands" is an array, and the "not" judgment does not apply to arrays (Use "any()" or "all()" operation for array judgment). The "not" judgment should be corrected to "is" judgment to solve this compatibility problem.

Author: wwlswj

spectral/graphics/graphics.py

@@ -537,7 +537,7 @@ def get_rgb_meta(source, bands=None, **kwargs):
         if k not in _get_rgb_kwargs:
             raise ValueError('Invalid keyword: {0}'.format(k))
 
-    if not bands:
+    if bands is None:
         bands = []
     if len(bands) not in (0, 1, 3):
         raise Exception("Invalid number of bands specified.")