Null MavenMetaData.Snapshot#timestamp causes maven parser failures.

[pway99]

This fix prevents a null timestamp from producing an invalid URI, resulting in MavenDownloadingExceptions and maven-parser failures.

org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.openrewrite.maven:rewrite-maven-plugin:4.39.0-SNAPSHOT:run (default-cli) on project ...: Execution default-cli of goal org.openrewrite.maven:rewrite-maven-plugin:4.39.0-SNAPSHOT:run failed: Failed to download dependencies for .../pom.xml:
</dependency>
  <!--~~(Unable to download POM. Tried repositories:
   https://...-snapshot: HTTP 404)~~>--><dependency> 
 at org.openrewrite.maven.MavenParser.parseInputs (MavenParser.java:127)
    at org.openrewrite.Parser.parse (Parser.java:41)
    at org.openrewrite.maven.MavenMojoProjectParser.parseMaven (MavenMojoProjectParser.java:309)
    at org.openrewrite.maven.MavenMojoProjectParser.listSourceFiles (MavenMojoProjectParser.java:119)
    at org.openrewrite.maven.AbstractRewriteMojo.listResults (AbstractRewriteMojo.java:250)

[sambsnyd]

Thanks @pway99 !