Make sure runfiles fingerprinting accounts for empty files

Previously, Runfiles#fingerprint (and actions that depend on it for their key) would collide when runfiles differed only by empty files. This is a correctness bug exercised by changes like https://github.com/bazelbuild/bazel/commit/3fc3ddbc0d2a51853c0966bfd41834765ea619a5.

RELNOTES: None
PiperOrigin-RevId: 214432124

src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java

diff --git a/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java b/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
index 2793953..e03edd5 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
@@ -1188,5 +1188,9 @@
       fp.addPath(artifact.getRootRelativePath());
       fp.addPath(artifact.getExecPath());
     }
+
+    for (String name : getEmptyFilenames()) {
+      fp.addString(name);
+    }
   }
 }