Fix native.existing_rule() when argument is an existing file

--
MOS_MIGRATED_REVID=127218963

src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java

diff --git a/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java b/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
index c05e178..9a575ed 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
@@ -886,7 +886,7 @@
   private static SkylarkDict<String, Object> targetDict(
       Target target, Location loc, Environment env)
       throws NotRepresentableException, EvalException {
-    if (target == null && !(target instanceof Rule)) {
+    if (target == null || !(target instanceof Rule)) {
       return null;
     }
     SkylarkDict<String, Object> values = SkylarkDict.<String, Object>of(env);

src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java
index 1a430ed..9f6d170 100644
--- a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java
@@ -475,6 +475,24 @@
   }
 
   @Test
+  public void testExistingRuleReturnNone() throws Exception {
+    scratch.file(
+        "test/rulestr.bzl",
+        "def test_rule(name, x):",
+        "  print(native.existing_rule(x))",
+        "  if native.existing_rule(x) == None:",
+        "    native.cc_library(name = name)");
+    scratch.file(
+        "test/BUILD",
+        "load('//test:rulestr.bzl', 'test_rule')",
+        "test_rule('a', 'does not exist')",
+        "test_rule('b', 'BUILD')");
+
+    assertNotNull(getConfiguredTarget("//test:a"));
+    assertNotNull(getConfiguredTarget("//test:b"));
+  }
+
+  @Test
   public void testGetRule() throws Exception {
     scratch.file("test/skylark/BUILD");
     scratch.file(