[crystal-lang] Resolve type check compile error in ApiError (#18759)

* Even with .nil? typecheck, compiler still fails due to .empty? call on nillable type. This appears a poor implementation issue by crystal-lang, but this explicit try avoids the issue for now.

* Made call more succinct.

Author: usiegj00

modules/openapi-generator/src/main/resources/crystal/api_error.mustache

@@ -21,7 +21,7 @@ module {{moduleName}}
       msg = ""
       msg = msg + "\nHTTP status code: #{code}" if @code
       msg = msg + "\nResponse headers: #{response_headers}" if @response_headers
-      if @message.nil? || @message.empty?
+      if @message.try &.empty?
         msg = msg + "\nError message: the server returns an error but the HTTP response body is empty."
       else
         msg = msg + "\nResponse body: #{@message}"

samples/client/petstore/crystal/src/petstore/api_error.cr

@@ -29,7 +29,7 @@ module Petstore
       msg = ""
       msg = msg + "\nHTTP status code: #{code}" if @code
       msg = msg + "\nResponse headers: #{response_headers}" if @response_headers
-      if @message.nil? || @message.empty?
+      if @message.try &.empty?
         msg = msg + "\nError message: the server returns an error but the HTTP response body is empty."
       else
         msg = msg + "\nResponse body: #{@message}"