Recently, I ran into an issue with a resource referece in an Urban Code Deploy (UCD) resource that did not yet exist. When the value, that doesn’t yet exist I found this a great tip (UCD Documentation).
Change from
${p:resource/value-not-yet-populated}
to
${p?:resource/value-not-yet-populated}
UCD replaces the missing value with an empty string. It saved my sanity and ease of use.