Make RegularFileArtifactValue#toString not crash if the digest is n…

…ull.

Observed NPE in a debugger that was trying to show a string representation.

Also make the string representation of the digest consistent with `RemoteFileArtifactValue`.

PiperOrigin-RevId: 595554648
Change-Id: I705ed0ffabf4bde81bb3b1f653f30b7565e65b82

src/main/java/com/google/devtools/build/lib/actions/FileArtifactValue.java

@@ -319,8 +319,8 @@ public static FileArtifactValue createProxy(byte[] digest) {
     return createForNormalFile(digest, /* proxy= */ null, /* size= */ 0);
   }
 
-  private static String bytesToString(byte[] bytes) {
-    return "0x" + BaseEncoding.base16().omitPadding().encode(bytes);
+  private static String bytesToString(@Nullable byte[] bytes) {
+    return bytes == null ? "null" : "0x" + BaseEncoding.base16().omitPadding().encode(bytes);
   }
 
   private static final class DirectoryArtifactValue extends FileArtifactValue {
@@ -541,7 +541,7 @@ public long getModifiedTime() {
     @Override
     public String toString() {
       return MoreObjects.toStringHelper(this)
-          .add("digest", BaseEncoding.base16().lowerCase().encode(digest))
+          .add("digest", bytesToString(digest))
           .add("size", size)
           .add("proxy", proxy)
           .toString();