In XQuery 3.1 & XPath 3.1, an InlineFunctionExpr has a FunctionBody, which is an EnclosedExpr, which can be just "{ }".

In XQuery, the semantics of this are addressed:
    [Definition: In an enclosed expression, the optional expression enclosed
    in curly braces is called the content expression.] If the content
    expression is omitted, the value of the content expression is ().

However, these  sentences occur in 3.9 Node Constructors, which does not exist in the XPath spec, and so the semantics are not defined in the XPath spec.

(Moreover, in the XQuery spec, it's odd that those sentences are in 3.9 Node Constructors, since EnclosedExprs are not specific to node constructors.)