do not populate api_id if it's empty string

library_generation/test/resources/goldens/.repo-metadata-empty-api-id-golden.json

@@ -0,0 +1,18 @@
+{
+  "api_shortname": "baremetalsolution",
+  "name_pretty": "Bare Metal Solution",
+  "product_documentation": "https://cloud.google.com/bare-metal/docs",
+  "api_description": "Bring your Oracle workloads to Google Cloud with Bare Metal Solution and jumpstart your cloud journey with minimal risk.",
+  "client_documentation": "https://cloud.google.com/java/docs/reference/google-cloud-bare-metal-solution/latest/overview",
+  "release_level": "preview",
+  "transport": "grpc",
+  "language": "java",
+  "repo": "googleapis/google-cloud-java",
+  "repo_short": "java-bare-metal-solution",
+  "distribution_name": "com.google.cloud:google-cloud-bare-metal-solution",
+  "api_id": "baremetalsolution.googleapis.com",
+  "library_type": "GAPIC_AUTO",
+  "requires_billing": true,
+  "rest_documentation": "https://cloud.google.com/bare-metal/docs/reference/rest",
+  "rpc_documentation": "https://cloud.google.com/bare-metal/docs/reference/rpc"
+}