build.plat: avoid type confusion in _check_feature.

Before this commit, `_check_feature(valid_xdrs=0)` would mean that
XDR buffers are not supported. Only `_check_feature(valid_xdrs=())`
was intended to be an indicator of that.

nmigen/build/plat.py

@@ -177,7 +177,7 @@ class Platform(ResourceManager, metaclass=ABCMeta):
                                   .format(type(self).__name__))
 
     def _check_feature(self, feature, pin, attrs, valid_xdrs, valid_attrs):
-        if not valid_xdrs:
+        if len(valid_xdrs) == 0:
             raise NotImplementedError("Platform '{}' does not support {}"
                                       .format(type(self).__name__, feature))
         elif pin.xdr not in valid_xdrs: