try to use empty GapicPackageInfo

Author: diegomarquezp

gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/ClientLibraryPackageInfoComposer.java

@@ -44,7 +44,9 @@ public class ClientLibraryPackageInfoComposer {
   private static final String SERVICE_DESCRIPTION_HEADER_PATTERN = "Service Description: %s";
 
   public static GapicPackageInfo generatePackageInfo(GapicContext context) {
-    Preconditions.checkState(!context.services().isEmpty(), "No services found to generate");
+    if (context.services().isEmpty()) {
+      return GapicPackageInfo.empty();
+    }
 
     // Pick some service's package, as we assume they are all the same.
     String libraryPakkage = context.services().get(0).pakkage();

gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicPackageInfo.java

@@ -26,6 +26,10 @@ public static GapicPackageInfo with(PackageInfoDefinition packageInfo) {
     return builder().setPackageInfo(packageInfo).build();
   }
 
+  public static GapicPackageInfo empty() {
+    return builder().setPackageInfo(PackageInfoDefinition.builder().build()).build();
+  }
+
   static Builder builder() {
     return new AutoValue_GapicPackageInfo.Builder();
   }

gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/Parser.java

@@ -176,13 +176,13 @@ public static GapicContext parse(CodeGeneratorRequest request) {
             transport);
 
     if (services.isEmpty()) {
-      //throw new NoServicesFoundException();
+      System.err.println("No services found. Will not generate Gapic clients");
     }
 
     // TODO(vam-google): Figure out whether we should keep this allowlist or bring
     // back the unused resource names for all APIs.
     // Temporary workaround for Ads, who still need these resource names.
-    if (services.get(0).protoPakkage().startsWith("google.ads.googleads.v")) {
+    if (!services.isEmpty() && services.get(0).protoPakkage().startsWith("google.ads.googleads.v")) {
       Function<ResourceName, String> typeNameFn =
           r -> r.resourceTypeString().substring(r.resourceTypeString().indexOf("/") + 1);
       Function<Set<ResourceName>, Set<String>> typeStringSetFn =