Convert single null argument to Optional.empty() in SpEL varargs expression

[sbrannen]

Overview

While testing the regression raised in #27719, I noticed that our support for java.util.Optional is inconsistent when invoking a method or function that accepts a varargs array of type Optional in a SpEL expression.

As can be seen in MethodInvocationTests, if null is supplied along with other values, each null value gets converted to Optional.empty(); whereas, a null value supplied by itself gets passed through as null to the invoked method.

spring-framework/spring-expression/src/test/java/org/springframework/expression/spel/MethodInvocationTests.java

Lines 305 to 310 in 62f480a

	// The following should actually evaluate to [Optional.empty] instead of [null],
	// but ReflectionHelper.convertArguments() currently does not provide explicit
	// Optional support for a single argument passed to a varargs array.
	evaluate("optionalVarargsMethod(null)", "[null]", String.class);
	evaluate("optionalVarargsMethod(null,'a')", "[Optional.empty, Optional[a]]", String.class);
	evaluate("optionalVarargsMethod('a',null,'b')", "[Optional[a], Optional.empty, Optional[b]]", String.class);

Although this is a very specific use case that potentially does not even exist in a real project, we would like to ensure consistent semantics for Optional arguments in SpEL expressions.