[join-] return the check for row not existing in sheet

After de0ac67, when sheet did not exist in row, None would get
returned as TypedExceptioNWrapper.

visidata/features/join.py

@@ -130,8 +130,13 @@ def __init__(self, name='', keycols=None, **kwargs):
         self.keycols = keycols
 
     def calcValue(self, row):
-        c = self.keycols[0]
-        return c.getTypedValue(row[c.sheet])
+        vals = set()
+        for i, c in enumerate(self.keycols):
+            if row[c.sheet] is not None:
+                vals.add(c.getTypedValue(row[c.sheet]))
+        if len(vals) != 1:
+            vd.warning(f'inconsistent keys: ' + str(vals))
+        return vals.pop()
 
     def putValue(self, row, value):
         for i, c in enumerate(self.keycols):