Honor empty <relativePath/> in MavenPomDownloader

[Jenson3210]

Summary

• Distinguish between omitted <relativePath> (defaults to ../pom.xml) and explicit empty <relativePath/> (skip local parent lookup) in MavenPomDownloader
• Fix affects both getParentWithinProject() and the relative-path resolution block in download()

Problem

When a POM declares <relativePath/> (empty self-closing tag), Maven skips local parent lookup entirely and goes straight to remote repositories (DefaultModelBuilder.java#L1048). OpenRewrite's MavenPomDownloader was treating empty relativePath the same as omitted because both null and "" pass through StringUtils.isBlank(), causing it to default to ../pom.xml in both cases.

Solution

Check for empty string explicitly before the isBlank fallback:

• null (element absent) → default to ../pom.xml
• "" (explicit empty <relativePath/>) → skip local parent lookup
• non-blank string → use that path

Test plan

• Existing tests pass (MavenPomDownloaderTest, MavenParserTest, ChangeParentPomTest, ResolvedPomTest)

• New test emptyRelativePathSkipsLocalParentLookup verifies the fix

• Fixes moderneinc/customer-requests#1950

[Jenson3210]

@MBoegers I could use your recent investigations to get bit more insights on this one.
Is this a valid change?

[MBoegers]

Added another test:

The test exercises these steps through download():

1. Stage 1 exact GAV lookup in projectPomsByGav: no match because ${my.version} != 1.0.0
2. Stage 2 iterate all project POMs, find test:parent, try to resolve the placeholder: fails because the parent doesn’t define my.version, so ${my.version} stays unresolved and doesn’t match 1.0.0
3. Stage 3 path-based lookup using relativePath:

• Without fix: empty relativePath defaults to .., resolves child/../pom.xml, finds the parent, and the ${ in the version relaxes the version check → returns local POM (wrong)
• With fix: empty relativePath means “skip local lookup” → block is skipped entirely
  Remote download — falls through to fetch from the nonexistent repo → throws exception

The assertThrows(Exception.class) confirms stage 3 was skipped — if it wasn’t, the local POM would be returned and no exception would be thrown.

[MBoegers]

LGTM with the additional test we should have both tracks covered