Add default value for webhook secret to prevent startup failure

Author: mbhave

src/main/resources/application.properties

@@ -4,4 +4,4 @@ spring.jackson.serialization.indent_output=true
 projects.github.org: spring-io
 projects.github.team: spring-team
 projects.github.accesstoken: ${projects-github-accessToken}
-
+projects.github.webhook-secret: ${projects-github-webhookSecret:changeme}