Fix NPE for Scala Gatling generator

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Fix the following NPE:

[main] INFO  o.o.c.languages.AbstractScalaCodegen - Environment variable SCALA_POST_PROCESS_FILE not defined so the Scala code may not be properly formatted. To define it, try 'export SCALA_POST_PROCESS_FILE=/usr/local/bin/scalafmt' (Linux/Mac)
[main] INFO  o.o.c.languages.AbstractScalaCodegen - NOTE: To enable file post-processing, 'enablePostProcessFile' must be set to `true` (--enable-post-process-file for CLI).
Exception in thread "main" java.lang.NullPointerException
	at org.openapitools.codegen.languages.ScalaGatlingCodegen.preprocessOpenAPI(ScalaGatlingCodegen.java:242)
	at org.openapitools.codegen.DefaultGenerator.configureGeneratorProperties(DefaultGenerator.java:183)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:854)
	at org.openapitools.codegen.cmd.Generate.run(Generate.java:349)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:62)

cc @clasnake (2017/07) @jimschubert (2017/09) @shijinkui (2018/01) @ramzimaalej (2018/03)