[Java-okhttp-gson] Do not set content-type if content type is null

[mvistein]

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PR contents

If an API does not define a custom ContentType, an empty Content-Type: header is generated. Some servers do not like this and respond with HTTP status 400. If the Content-Type header is ommited completely in this situation, the connection is working again.

In PR #11199 a fix was attempted, however I do not think it solves the underlying problem. I think the main problem is a typo, which mixed up the variables localVarHeaderParams and localVarContentType. The selectHeaderContentType method return null if there is localVarContentTypes is empty and no default content type should be used.
The expression localVarContentType != null is always true, since the variable is properly initialized some lines earlier. The same goes for localVarContentTypes != null.

[wing328]

cc @MarcMil as well (author of #11199)

[wing328]

@mvistein thanks for the fix. I think you're right. localVarContentType should be used instead.