@Nullable after @JsonProperty

A Checkstyle's Regex rule could be added to avoid putting `@Nullable` before `@JsonProperty`, like `@Nullable @JsonProperty("foo") Foo foo`.

Putting `@Nullable` immediately before the type of the variable is more semantic because `@Nullable Foo` is an "extended type" of a variable.

From [here](https://github.com/apache/incubator-druid/pull/8209#discussion_r311674741).

No file or test is named. Start by locating Druid's Checkstyle configuration and existing Regex rules, then verify how the example declaration is matched. Done means the configuration rejects @Nullable before @JsonProperty while allowing the semantic placement immediately before the variable type.

Written by the indexing model from the issue text.