fix(gnovm): Inconsistency in the single-linked list implementation

[Davphla]

Fix: #4786

This PR simplify the implementation of the MapList linked list of the gnovm.
There's no semantic change as it is explicit in the original issue, it only simplify the code by expliciting the two cases:

• Empty list -> Set tail and head with item
• Append -> Set tail->next with item

[Gno2D2]

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