We don't document a default for the 's' format, if the user enters key_format=s, there's no default size documented, I'm guessing it should be 1, the same as the t format.

Setting key_format=s during the create succeeds, as does the WT_CURSOR.set_key, but the subsequent WT_CURSOR.insert fails with

cursor.set_key: Empty keys not permitted: Invalid argument

Additionally, we don't currently error if the application specifies key_format=0t or key_format=0s, the code resets the size to 1 in this case (not sure that's worth fixing, though).