Crash when using a proxy without a user

[jonenst]

• Do you want to request a feature or report a bug?
  bug

• What is the current behavior?
  The gse crashes during startup if a proxy config without a user/password is given

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
  Specify a proxy in config.yaml without a user:

proxy:    
  host: localhost                                                                           
  port: 3128         

or with a null value:

proxy:    
  host: localhost                                                                           
  port: 3128 
  user: ~
  password: ~        

equivalent to

proxy:    
  host: localhost                                                                           
  port: 3128 
  user: 
  password: 

The app crashes with the following stacktrace:
com.powsybl.commons.PowsyblException: Property user is not set
at com.powsybl.commons.config.AbstractModuleConfig.createPropertyNotSetException(AbstractModuleConfig.java:29)
at com.powsybl.commons.config.AbstractModuleConfig.lambda$getStringProperty$0(AbstractModuleConfig.java:34)
at java.util.Optional.orElseThrow(Optional.java:290)
at com.powsybl.commons.config.AbstractModuleConfig.getStringProperty(AbstractModuleConfig.java:34)
at com.powsybl.gse.util.GseUtil.lambda$setProxy$8(GseUtil.java:196)
at java.util.Optional.ifPresent(Optional.java:159)
at com.powsybl.gse.util.GseUtil.setProxy(GseUtil.java:193)
at com.powsybl.gse.app.GseApp.main(GseApp.java:94)
at com.powsybl.gse.demo.GseDemo.main(GseDemo.java:60)

• What is the expected behavior?
  Without a user password, the code should just assume an unauthenticated proxy

• What is the motivation / use case for changing the behavior?
  don"t force user to invent fake usernames/passwords

• Please tell us about your environment:

  • PowSyBl Version: 2.4, GSE 1.4

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)
  We should just change GseUtils.java public static void setProxy() to use getOptionalConfig, and create the authenticator only if they are set.