correctly resolve generic return types

when using the generator on a class with a generic return type a NullPointerException is thrown because "TypeVariable" is not resolved in ClassUtil.java. a generic return type is never null.

```
Full error stack : java.lang.NullPointerException
	at org.assertj.assertions.generator.description.converter.ClassToClassDescriptionConverter.getTypeDescription(ClassToClassDescriptionConverter.java:128)
	at org.assertj.assertions.generator.description.converter.ClassToClassDescriptionConverter.doGetterDescriptionsOf(ClassToClassDescriptionConverter.java:75)
	at org.assertj.assertions.generator.description.converter.ClassToClassDescriptionConverter.getterDescriptionsOf(ClassToClassDescriptionConverter.java:62)
	at org.assertj.assertions.generator.description.converter.ClassToClassDescriptionConverter.convertToClassDescription(ClassToClassDescriptionConverter.java:51)
	at org.assertj.maven.generator.AssertionsGenerator.generateAssertionsFor(AssertionsGenerator.java:70)
	at org.assertj.maven.AssertJAssertionsGeneratorMojo.executeWithAssertionGenerator(AssertJAssertionsGeneratorMojo.java:96)
	at org.assertj.maven.AssertJAssertionsGeneratorMojo.execute(AssertJAssertionsGeneratorMojo.java:88)
```