Fix NullPointerException in SuffixMapping.getTargetFiles

[JRVector9]

SuffixMapping.getTargetFiles() calls source.endsWith(sourceSuffix) unconditionally. Both source and sourceSuffix can be null through the public constructor (SuffixMapping(String sourceSuffix, String targetSuffix) takes no null checks), and either one being null throws an NPE:

java.lang.NullPointerException: Cannot invoke "String.endsWith(String)" because "source" is null
	at org.apache.maven.shared.io.scan.mapping.SuffixMapping.getTargetFiles(SuffixMapping.java:59)

java.lang.NullPointerException: Cannot invoke "String.length()" because "suffix" is null
	at java.base/java.lang.String.endsWith(String.java:2331)
	at org.apache.maven.shared.io.scan.mapping.SuffixMapping.getTargetFiles(SuffixMapping.java:59)

Fixes #93

Changes

• Guard the endsWith call so a null source or sourceSuffix is treated as a non-match (returns an empty target set), matching the existing behavior for a suffix mismatch.
• Added two tests (shouldReturnNoTargetFilesWhenSourceSuffixIsNull, shouldReturnNoTargetFilesWhenSourceIsNull) that reproduce the NPE before the fix and pass after it.

mvn test -Dtest=SuffixMappingTest and mvn verify pass locally.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

[elharo]

I suspect the correct fix is to throw NullPointerExceptions from the constructor. What does a null suffix mean?