Throw MavenParsingException when a POM is missing groupId or version

[steve-aom-elliott]

What

RawPom.toPom calls Objects.requireNonNull on the project's groupId and version (RawPom.java:437,439). When either is absent, the resulting NullPointerException propagates out of mod build / MavenParser.parseInputs with no artifact context, only this stack:

Caused by: java.lang.NullPointerException
    at java.util.Objects.requireNonNull (Objects.java:203)
    at org.openrewrite.maven.internal.RawPom.toPom (RawPom.java:437)
    at org.openrewrite.maven.internal.MavenPomDownloader.download (MavenPomDownloader.java:595)
    at org.openrewrite.maven.tree.ResolvedPom.resolveDependencies (ResolvedPom.java:1050)
    ...

There is no signal as to which transitive dependency's POM was malformed.

Why

A POM can legitimately omit a <groupId> or <version> element (relying on <parent> inheritance), but it can also be invalid — for example:

• a stub POM uploaded to an internal Artifactory/Nexus alongside a vendored JAR via mvn install:install-file or REST upload (which bypasses ModelValidator)
• a POM emitted by non-Maven tooling that wrote <groupId/> or <groupId></groupId> instead of omitting the element (Jackson-XML's EMPTY_ELEMENT_AS_NULL deserializes those to null)
• a POM with whitespace-only content like <version> </version> (which MavenXmlMapper's StringTrimModule reduces to "", then sailed past every downstream null-check)

In each of these cases the resolver cannot construct a usable Pom, but the user has no way of telling which artifact is the source.

How

Two contained changes inside RawPom:

1. getGroupId() and getVersion() now treat blank (null or whitespace-only) values as missing, so an empty <groupId/> or <version> </version> is treated the same as an omitted element. Scope is limited to these two methods; nothing outside RawPom is affected.

2. toPom() checks the resolved coordinates and throws MavenParsingException (already used at sibling validation points in ResolvedPom) naming the artifact and source repository:

   POM is missing a required coordinate: groupId for {null:my-app:1} (from https://repo.example.com/...)
   

MavenParsingException is unchecked, so no signature changes ripple out to MavenPomDownloader, MavenParser, Assertions, or tests.

Tests

Added five tests to RawPomTest:

• missingGroupIdThrowsParsingException
• missingVersionThrowsParsingException
• emptyGroupIdElementThrowsParsingException
• whitespaceVersionElementThrowsParsingException
• groupIdInheritedFromParentStillResolves (regression check that legitimate parent inheritance still works)

[timtebeek]

Great to capture these with more details, thanks!