Closes gh-23934

...or skip if there are no values either.

Closes gh-22372

Closes gh-1956

See gh-1956

Closes gh-23984

* https://github.com/spring-projects/spring-framework/blob/3a0f309e2c9fdbbf7fb2d348be861528177f8555/spring-web/src/main/java/org/springframework/http/codec/ServerSentEvent.java#L24
* https://github.com/spring-projects/spring-framework/blob/3a0f309e2c9fdbbf7fb2d348be861528177f8555/spring-web/src/main/java/org/springframework/http/codec/ClientCodecConfigurer.java#L88
* https://github.com/spring-projects/spring-framework/blob/ef14d76d3637abeb31edd8a22031c21f9445efef/spring-web/src/main/java/org/springframework/http/codec/ServerCodecConfigurer.java#L94
* https://github.com/spring-projects/spring-framework/blob/3a0f309e2c9fdbbf7fb2d348be861528177f8555/spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/handler/EventSourceTransportHandler.java#L33

Prior to this commit, the schemaZip Gradle task failed to find Spring
schema files on MS Windows due to path separators hard coded to forward
slashes that are not compatible with the Windows operating system.

Consequently, a full build failed on Windows since the distZip task was
not able to locate the zipped schema archive that the schemaZip task
failed to create.

This commit fixes this by updating the schemaZip task to search for
schema files using backslashes as well as forward slashes.

Closes gh-23933

Closes gh-23812

Closes gh-23977

Closes gh-23978

Closes gh-23980

See gh-23296

See gh-23296

Closes gh-23929

See gh-23784

Prior to this commit, the decoder did not allow for a null value
returned from the Jackson object reader.

Closes: gh-23935

Javadoc indicates the returned value can be null.

Closes gh-2023

See gh-2023

Use when xpath result is XPathConstants.NODESET

Closes gh-2024

Closes gh-23972

Closes gh-23976

Currently, if you have an optional event listener (via a @Bean method returning `null`)
this causes the event multicaster to explode violently.  Now, we just safely skip it.

Closes gh-23958

See gh-23970

Previously, spring-webmvc and spring-webflux both contained tests
that would create gzipped files, write them to the filesystem
alongside the project's compiled test classes, and configure them to
be deleted on JVM exit. The output location placed the files on the
classpath, polluting it for every subsequent test that used the same
ClassLoader. The test-sources plugin combined with Gradle's use of
worker JVMs, broadens the scope of this pollution to other, downstream
projects in the same build. For example, the tests for
spring-websocket will have a different classpath depending on whether
or not the tests for spring-webmvc have already been run on the same
worker as part of the current build.

This commit updates the spring-webmvc and spring-webflux modules to
introduce a new JUnit Jupiter extension, GzipSupport. This extension
allows gzipped files to be created via an injectable GzippedFiles
class and automatically deletes each created file in an after-each
callback. This ensures that a gzipped file only exists on the
classpath for the duration of the test that needs it, avoiding the
pollution of the classpath of any subsequent tests.

Closes gh-23970

Closes gh-23971

Closes gh-23409

Closes gh-23954

Closes gh-23956