Spring SpEL cannot invoke varargs MethodHandle function with zero variable arguments

The Spring Expression Language (SpEL) currently cannot invoke a varargs MethodHandle function with zero variable arguments.

Attempting to do so results in a SpelEvaluationException with an INCORRECT_NUMBER_OF_ARGUMENTS_TO_FUNCTION message.

For example, given a MethodHandle registered as a function named format that accepts (String, Object...), the SpEL expression #format('x', 42) is supported, but the expression #format('x') fails even though the variable arguments are not required.

The following assertion in our test suite should therefore not actually fail due to a SpelEvaluationException with an INCORRECT_NUMBER_OF_ARGUMENTS_TO_FUNCTION message but rather due to an exception thrown from java.lang.String.formatted(Object...), which that #message() function invokes on the first argument.

https://github.com/spring-projects/spring-framework/blob/c6b20c04c24aa4689f7c5387bdf141301ab989d5/spring-expression/src/test/java/org/springframework/expression/spel/VariableAndFunctionTests.java#L59