Table: check for missing X only if not sparse

Missing entries in sparse are considered to be 0.

Orange/data/table.py

@@ -982,7 +982,8 @@ def total_weight(self):
 
     def has_missing(self):
         """Return `True` if there are any missing attribute or class values."""
-        return bn.anynan(self.X) or bn.anynan(self._Y)
+        missing_x = not sp.issparse(self.X) and bn.anynan(self.X)   # do not check for sparse X
+        return missing_x or bn.anynan(self._Y)
 
     def has_missing_class(self):
         """Return `True` if there are any missing class values."""