admin: remove compulsory admin message in bootstrap. (#4726)

Now that Envoy can function without an admin listener (#4486), there's no need to specify path or
even an empty admin message, so relax this requirement.

Risk Level: Low
Testing: bazel test //test/...

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 570a403b8882ea34309bbe73209fee41417fa10e

envoy/config/bootstrap/v2/bootstrap.proto

@@ -134,8 +134,9 @@ message Bootstrap {
 // <operations_admin_interface>`.
 message Admin {
   // The path to write the access log for the administration server. If no
-  // access log is desired specify ‘/dev/null’.
+  // access log is desired specify ‘/dev/null’. This is only required if
-  string access_log_path = 1 [(validate.rules).string.min_bytes = 1];
+  // :ref:`address <envoy_api_field_config.bootstrap.v2.Admin.address>` is set.
+  string access_log_path = 1;
 
   // The cpu profiler output path for the administration server. If no profile
   // path is specified, the default is ‘/var/log/envoy/envoy.prof’.