* Add failing tests for property placeholder in encoding configuration

When the maven-compiler-plugin encoding is configured with a property placeholder
like ${java.encoding}, getCharset() throws IllegalCharsetNameException because
Charset.forName() receives the unresolved placeholder string.

* Skip unresolved property placeholders when determining charset

When the maven-compiler-plugin encoding is configured with a property placeholder
like ${java.encoding}, getCharset() would throw IllegalCharsetNameException because
Charset.forName() received the unresolved placeholder string.

This follows the same pattern used elsewhere in this class for handling unresolved
properties (e.g., release, source, target settings) - skip values containing "${"
and let the method return Optional.empty() instead.

Also fixed a potential NPE when getPluginManagement() returns null.

Fixes #1104

* Resolve property placeholders from Maven properties

Instead of just skipping unresolved property placeholders, attempt to resolve
them from MavenProject.getProperties(). This allows configurations like
${java.encoding} to work when the property is defined in the project.

---------

Co-authored-by: Tim te Beek <[email protected]>

Bumps [org.apache.maven.plugins:maven-compiler-plugin](https://github.com/apache/maven-compiler-plugin) from 3.14.1 to 3.15.0.
- [Release notes](https://github.com/apache/maven-compiler-plugin/releases)
- [Commits](apache/maven-compiler-plugin@maven-compiler-plugin-3.14.1...maven-compiler-plugin-3.15.0)

---
updated-dependencies:
- dependency-name: org.apache.maven.plugins:maven-compiler-plugin
  dependency-version: 3.15.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Use this link to re-run the recipe: https://app.moderne.io/recipes/org.openrewrite.DeleteSourceFiles?organizationId=QUxML01vZGVybmUvTW9kZXJuZSArIE9wZW5SZXdyaXRl#defaults=W3sibmFtZSI6ImZpbGVQYXR0ZXJuIiwidmFsdWUiOiIuZ2l0aHViL3dvcmtmbG93cy8qLXByLnltbCJ9XQ==

Co-authored-by: Moderne <[email protected]>

Use this link to re-run the recipe: https://app.moderne.io/recipes/org.openrewrite.recipes.rewrite.OpenRewriteRecipeBestPractices?organizationId=QUxML01vZGVybmUvTW9kZXJuZSArIE9wZW5SZXdyaXRl

Co-authored-by: Moderne <[email protected]>