fix file checking logic.

Author: rmehta19

gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java

@@ -502,7 +502,7 @@ ChannelCredentials createS2ASecuredChannelCredentials() {
     // Try to load MTLS-MDS creds.
     File rootFile = new File(MTLS_MDS_ROOT);
     File certKeyFile = new File(MTLS_MDS_CERT_CHAIN_AND_KEY);
-    if (!rootFile.isFile() || !certKeyFile.isFile()) {
+    if (rootFile.isFile() && certKeyFile.isFile()) {
       // Try to connect to S2A using mTLS.
       ChannelCredentials mtlsToS2AChannelCredentials = null;
       try {