Do not crash on URIs without a host component.

URIs such as `unix://...` (Unix domain socket) may legitimately be missing a
host component. We should gracefully handle them when looking for a credential
helper (no such helper can exist for them since the lookup is host-based, but
this could change in the future).

Fixes #16171 which is a regression in 5.3. We had some tests for this in
remote_execution_test.sh, but they were disabled due to flakiness in 8e3c0df.
I've filed #16185 to reenable them.

src/main/java/com/google/devtools/build/lib/authandtls/credentialhelper/CredentialHelperProvider.java

@@ -66,7 +66,12 @@ private CredentialHelperProvider(
   public Optional<CredentialHelper> findCredentialHelper(URI uri) {
     Preconditions.checkNotNull(uri);
 
-    String host = Preconditions.checkNotNull(uri.getHost());
+    String host = uri.getHost();
+    if (host == null) {
+      // Some URIs (e.g. unix://) legitimately have no host component.
+      return Optional.empty();
+    }
+
     Optional<Path> credentialHelper =
         findHostCredentialHelper(host)
             .or(() -> findWildcardCredentialHelper(host))