Add a sanity check to prevent creation of Artifacts with an empty path

And fix the tests that were doing this.

PiperOrigin-RevId: 179548691

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

diff --git a/src/main/java/com/google/devtools/build/lib/actions/ActionInputHelper.java b/src/main/java/com/google/devtools/build/lib/actions/ActionInputHelper.java
index 49e262f..0f2c387 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/ActionInputHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/ActionInputHelper.java
@@ -72,6 +72,7 @@
 
     public BasicActionInput(String path) {
       this.path = Preconditions.checkNotNull(path);
+      Preconditions.checkArgument(!path.isEmpty());
     }
 
     @Override

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

diff --git a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
index 7d1a5b7..7ecdede 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
@@ -189,6 +189,10 @@
       throw new IllegalArgumentException(execPath + ": illegal execPath for " + path
           + " (root: " + root + ")");
     }
+    if (execPath.segmentCount() == 0) {
+      throw new IllegalArgumentException(
+          "it is illegal to create an artifact with an empty execPath");
+    }
     this.hashCode = path.hashCode();
     this.path = path;
     this.root = root;