Use fallback build id if jar version is null

Bug: 27530700
Change-Id: Idc8c7f22a4f261b810d87ab128e64c6d8fef5933
diff --git a/common/host-side/tradefed/src/com/android/compatibility/common/tradefed/build/CompatibilityBuildProvider.java b/common/host-side/tradefed/src/com/android/compatibility/common/tradefed/build/CompatibilityBuildProvider.java
index 3b1ca25..f38517b 100644
--- a/common/host-side/tradefed/src/com/android/compatibility/common/tradefed/build/CompatibilityBuildProvider.java
+++ b/common/host-side/tradefed/src/com/android/compatibility/common/tradefed/build/CompatibilityBuildProvider.java
@@ -37,8 +37,11 @@
     @Override
     public IBuildInfo getBuild() {
         // Create a blank BuildInfo which will get populated later.
-        IBuildInfo ctsBuild = new BuildInfo(
-                Package.getPackage(PACKAGE_NAME).getImplementationVersion(), "cts", "cts");
+        String version = Package.getPackage(PACKAGE_NAME).getImplementationVersion();
+        if (version == null) {
+            version = IBuildInfo.UNKNOWN_BUILD_ID;
+        }
+        IBuildInfo ctsBuild = new BuildInfo(version, "cts", "cts");
         if (mBranch  != null) {
             ctsBuild.setBuildBranch(mBranch);
         }