Fixes https://github.com/spring-projects/spring-integration/issues/3114
The contract of SpEL with its
`getValue(EvaluationContext context, @Nullable Object rootObject)` is
that we need to deal with provided `rootObject` even if it is `null`
and don't consult with `context.getRootObject()`
* Fix `ExpressionEvalMap` to have an internal `rootExplicitlySet`
to indicate that `root` explicitly provided by consumer, even if it is null.
According this flag call respective `Expression.getValue()`
* Add `@Nullable` to methods and their arguments into `ExpressionEvalMap`
& `FunctionExpression` to honor `Expression` contracts
* Populate an `HttpEntity` explicitly into `ExpressionEvalMap` from the
`HttpRequestHandlingEndpointSupport` and `WebFluxInboundEndpoint` for
full picture
* Fix JavaDocs indentations in the `ExpressionEvalMap`
516ecbcf8b