GH-3114: Honor SpEL contract in ExpressionEvalMap

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`

spring-integration-http/src/main/java/org/springframework/integration/http/inbound/HttpRequestHandlingEndpointSupport.java

@@ -276,6 +276,7 @@ public abstract class HttpRequestHandlingEndpointSupport extends BaseHttpInbound
 				headers.putAll(
 						ExpressionEvalMap.from(getHeaderExpressions())
 								.usingEvaluationContext(evaluationContext)
+								.withRoot(httpEntity)
 								.build());
 			}