Fixes NullPointerException in pkgcaches_test on ci.bazel.io

Fixes #879.

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MOS_MIGRATED_REVID=114728312

src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java

diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java b/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java
index 4bf976a..6126197 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java
@@ -85,9 +85,11 @@
 
     // The outputBase may be null if we're not actually running a build.
     Path outputBase = pkgLocator.get().getOutputBase();
-    Path relativeExternal = pkgLocator
-        .get().getOutputBase().getRelative(Label.EXTERNAL_PATH_PREFIX);
-    if (outputBase != null && !rootedPath.asPath().startsWith(relativeExternal)) {
+    if (outputBase == null) {
+      return;
+    }
+    Path relativeExternal = outputBase.getRelative(Label.EXTERNAL_PATH_PREFIX);
+    if (!rootedPath.asPath().startsWith(relativeExternal)) {
       return;
     }