DMN compiler throws NPE when underlying compilation fails and returns null

When an underlying compilation fails and returns null, the compiler throws NPE. See the code here [1]. I will provide a PR with a fix and a reproducer.

[1] https://github.com/baldimir/drools/blob/7a6db1342925872a898dee3f1d03f835b198a7eb/kie-dmn/kie-dmn-core/src/main/java/org/kie/dmn/core/compiler/DMNCompilerImpl.java#L116