Better documentation of the method_name field.

Also passing empty string as opposed to NULL when the method name is not
found. This is much less error-prone.

include/grpc/grpc_security.h

@@ -247,7 +247,8 @@ typedef struct {
   const char *service_url;
 
   /* The method name of the RPC being called (not fully qualified).
-     Can be NULL if no method name was found. */
+     The fully qualified method name can be built from the service_url:
+     full_qualified_method_name = ctx->service_url + '/' + ctx->method_name. */
   const char *method_name;
 
   /* The auth_context of the channel which gives the server's identity. */

src/core/security/client_auth_filter.c

@@ -146,6 +146,7 @@ void build_auth_metadata_context(grpc_security_connector *sc,
     *last_slash = '\0';
     method_name = gpr_strdup(last_slash + 1);
   }
+  if (method_name == NULL) method_name = gpr_strdup("");
   gpr_asprintf(&service_url, "%s://%s%s",
                sc->url_scheme == NULL ? "" : sc->url_scheme,
                grpc_mdstr_as_c_string(calld->host), service);