Fix profile picture retrieval without key

[wjans]

Profile pictures should not be fetched if the key is null.

java.lang.IllegalArgumentException: Key cannot be null
	at com.amazonaws.util.ValidationUtils.assertNotNull(ValidationUtils.java:37) ~[aws-java-sdk-core-1.11.125.jar:na]
	at com.amazonaws.util.ValidationUtils.assertStringNotEmpty(ValidationUtils.java:87) ~[aws-java-sdk-core-1.11.125.jar:na]
	at com.amazonaws.services.s3.AmazonS3Client.getObject(AmazonS3Client.java:1330) ~[aws-java-sdk-s3-1.11.125.jar:na]
	at com.amazonaws.services.s3.AmazonS3Client.getObject(AmazonS3Client.java:1243) ~[aws-java-sdk-s3-1.11.125.jar:na]
	at com.ixortalk.aws.s3.library.config.AwsS3Template.get(AwsS3Template.java:58) ~[ixortalk-aws-s3-library-1.1.0.jar:1.1.0]
	at com.ixortalk.authserver.web.rest.UserProfilePictureResource.lambda$getProfilePicture$0(UserProfilePictureResource.java:109) ~[classes/:na]
	at java.util.Optional.map(Optional.java:215) ~[na:1.8.0_181]
	at com.ixortalk.authserver.web.rest.UserProfilePictureResource.getProfilePicture(UserProfilePictureResource.java:107) ~[classes/:na]
	at com.ixortalk.authserver.web.rest.UserProfilePictureResource.getProfilePictureByLogin(UserProfilePictureResource.java:95) ~[classes/:na]```