RATIS-1322. Cannot build from source without repo

[adoroszlai]

What changes were proposed in this pull request?

Fix the build error that occurs if Ratis directory is not a git repo (which would happen when building from release source artifact):

[INFO] Apache Ratis ....................................... FAILURE [  0.819 s]
...
[ERROR] Failed to execute goal org.codehaus.mojo:buildnumber-maven-plugin:1.4:create-metadata (default) on project ratis: Execution default of goal org.codehaus.mojo:buildnumber-maven-plugin:1.4:create-metadata failed.: NullPointerException -> [Help 1]

revision in ratis-version.properties is set to Unknown in this case. This matches Ozone's properties for the same situation.

https://issues.apache.org/jira/browse/RATIS-1322

How was this patch tested?

$ mvn -DskipTests clean verify; cat target/classes/ratis-version.properties
...
version=1.1.0-SNAPSHOT
revision=1ce8621b295678c780ad7adaf5621d8114fdefc3
name=Apache Ratis
timestamp=1614187092964

$ rm -fr .git
$ mvn -DskipTests clean verify; cat target/classes/ratis-version.properties
...
version=1.1.0-SNAPSHOT
revision=Unknown
name=Apache Ratis
timestamp=1614187444515