[JXR-170] NullPointerException while parsing Java 15 multi-line String source

[spannm]

This PR offers a fix for bug [JXR-170]. Summary of changes:

• Add support for parsing sources with Java 15 multi-line Strings
• Add JUnit test case
• Implement toString on class BaseType
• Require non-null name on BaseType construtor
• Prevent construction of types with null name
• Add version to maven-enforcer-plugin plugin to prevent failure of integration test JXR-143_nofork

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[slawekjaranowski]

@sman-81 thanks

[spannm]

@sman-81 thanks

Thank you, too @slawekjaranowski