Fix NullPointerException with oneOf discriminator change

[jmini]

While I was working on #29, I have found a NullPointerException:

java.lang.NullPointerException
	at com.qdesrame.openapi.diff.model.ChangedSchema.isChanged(ChangedSchema.java:76)
	at com.qdesrame.openapi.diff.model.Changed.isUnchanged(Changed.java:15)
	at com.qdesrame.openapi.diff.model.ChangedMediaType.isChanged(ChangedMediaType.java:23)
	at com.qdesrame.openapi.diff.model.Changed.isUnchanged(Changed.java:15)
	at com.qdesrame.openapi.diff.utils.ChangedUtils.isUnchanged(ChangedUtils.java:10)
	at com.qdesrame.openapi.diff.compare.ContentDiff.diff(ContentDiff.java:40)
	at com.qdesrame.openapi.diff.compare.RequestBodyDiff.computeDiff(RequestBodyDiff.java:61)
	at com.qdesrame.openapi.diff.compare.RequestBodyDiff.computeDiff(RequestBodyDiff.java:1)
	at com.qdesrame.openapi.diff.compare.ReferenceDiffCache.cachedDiff(ReferenceDiffCache.java:48)
	at com.qdesrame.openapi.diff.compare.RequestBodyDiff.diff(RequestBodyDiff.java:30)
	at com.qdesrame.openapi.diff.compare.OperationDiff.diff(OperationDiff.java:32)
	at com.qdesrame.openapi.diff.compare.PathDiff.diff(PathDiff.java:34)
	at com.qdesrame.openapi.diff.compare.PathsDiff.lambda$0(PathsDiff.java:59)
	at java.util.LinkedHashMap$LinkedKeySet.forEach(LinkedHashMap.java:559)
	at com.qdesrame.openapi.diff.compare.PathsDiff.diff(PathsDiff.java:39)
	at com.qdesrame.openapi.diff.compare.OpenApiDiff.compare(OpenApiDiff.java:94)
	at com.qdesrame.openapi.diff.compare.OpenApiDiff.compare(OpenApiDiff.java:67)
	at com.qdesrame.openapi.diff.OpenApiCompare.fromSpecifications(OpenApiCompare.java:99)
	at com.qdesrame.openapi.diff.OpenApiCompare.fromLocations(OpenApiCompare.java:88)
	at com.qdesrame.openapi.diff.OpenApiCompare.fromLocations(OpenApiCompare.java:76)

This PR fix it an add a test.

[quen2404]

Thank you for this pull request ! 👍