Fix empty value

[paderlol]

In the application file, when using the environment variable expression, not adding the default null expression causes the program to start without an error because of this value

Incorrect

nacos.core.auth.plugin.nacos.token.secret.key=${NACOS_AUTH_TOKEN}
nacos.core.auth.server.identity.key=${NACOS_AUTH_IDENTITY_KEY}
nacos.core.auth.server.identity.value=${NACOS_AUTH_IDENTITY_VALUE}

correct

nacos.core.auth.plugin.nacos.token.secret.key=${NACOS_AUTH_TOKEN:}
nacos.core.auth.server.identity.key=${NACOS_AUTH_IDENTITY_KEY:}
nacos.core.auth.server.identity.value=${NACOS_AUTH_IDENTITY_VALUE:}