cucumber / cucumber-jvm / 3652

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[Java] Support empty strings and null values in data tables (#1857)

Empty data table cells can either be considered null or empty strings.
For example table below can converted to a map as
`{name=Aspiring Author, first publication=}` or
`{name=Aspiring Author, first publication=null}`.

```gherkin
 | name            | first publication |
 | Aspiring Author |                   |
```

And as demonstrated #1617 there are good reasons to default the empty
table cell to null. However this does not cover all cases. There are
however good use cases to use the empty string.

By declaring a table transformer with a replacement string it becomes
possible to explicitly disambiguate between the two scenarios. For
example:

```gherkin
Given some authors
   | name            | first publication |
   | Aspiring Author |                   |
   | Ancient Author  | [blank]           |
```

```java
@DataTableType(replaceWithEmptyString = "[blank]")
public Author convert(Map<String, String> entry){
  return new Author(
     entry.get("name"),
     entry.get("first publication")
  );
}

@Given("some authors")
public void given_some_authors(List<Author> authors){
  // authors = [Author(name="Aspiring Author", firstPublication=null), Author(name="Ancient Author", firstPublication=)]
}
```

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