Allow null as leftNullValue/rightNullValue in Join.fullOuterJoin()

When using the [Join.fullOuterJoin()](https://beam.apache.org/documentation/sdks/javadoc/2.4.0/org/apache/beam/sdk/extensions/joinlibrary/Join.html#fullOuterJoin-org.apache.beam.sdk.values.PCollection-org.apache.beam.sdk.values.PCollection-V1-V2-), a checkNotNull() is done for the [leftNullValue](https://github.com/apache/beam/blob/master/sdks/java/extensions/join-library/src/main/java/org/apache/beam/sdk/extensions/joinlibrary/Join.java#L207) and [rightNullValue](https://github.com/apache/beam/blob/master/sdks/java/extensions/join-library/src/main/java/org/apache/beam/sdk/extensions/joinlibrary/Join.java#L208).

However, it makes more sense to allow null values, since sometimes, if the key used for the join is not the same, you'd like to see that the value will become null. This should be decided by the developer, and not by the join library.

Looking at the source code, this is also supported by [KV.of()](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/values/KV.java#L42) (it allows null values), which is used in Join.fullOuterJoin().

If required, I can create a pull request on GitHub.

Imported from Jira [BEAM-4114](https://issues.apache.org/jira/browse/BEAM-4114). Original Jira may contain additional context.
Reported by: rtrietsch.