You can use a conditional to check if the field does not exist and drop it, just put it after the grok filter where you create the field.
if ![jenkins_build_number] {
drop {}
}
But since you create it in a grok filter and you do not have the field in some message, they probably have the tag _grokparsefailure, which you can also use to drop messages.
if "_grokparsefailure" in [tags] {
drop {}
}