Fix for uncompilable java client if no model definitions i specified (#7836)

After fixes for #6636 the gson based clients would not compile if the
swagger json did not have any model definitions. This can be the case
if the API only uses simple datastructures for input and output. (int,
String, List, Map etc.)

Author: eivinhb

modules/swagger-codegen/src/main/resources/Java/JSON.mustache

@@ -25,7 +25,9 @@ import org.threeten.bp.OffsetDateTime;
 import org.threeten.bp.format.DateTimeFormatter;
 {{/threetenbp}}
 
+{{#parent.length}}
 import {{modelPackage}}.*;
+{{/parent.length}}
 import okio.ByteString;
 
 import java.io.IOException;

modules/swagger-codegen/src/main/resources/Java/libraries/retrofit2/JSON.mustache

@@ -25,7 +25,9 @@ import org.threeten.bp.OffsetDateTime;
 import org.threeten.bp.format.DateTimeFormatter;
 {{/threetenbp}}
 
+{{#parent.length}}
 import {{modelPackage}}.*;
+{{/parent.length}}
 
 import java.io.IOException;
 import java.io.StringReader;