I think 10 is an even and good score to use for unknown NPC attributes, except for strength and agility that sometimes can have 13 or 17 based on damage bonus.
Agree there still needs to be a default attribute and your suggestion goes part of the way. You can also have higher WILL (which will show up in higher WP) and CON (higher HP), which may be relevant (e.g. CON is used to resist poisons), so it’s not just STR and AGL that can be above the average, e.g. a guard has a damage bonus of STR +d4 so must have min STR 13, and 12 HP and no Robust HA so must have a CON of 12. All other attributes should default. Higher attributes can also lead to higher skill bases than the default of 5, but not sure it’s worth worrying about as you can list specific skills.
The more I think about it, the more I think just having the attributes on the NPC stat block would be cleaner then some rule text talking about defaulting when something isn’t listed, but I think there needs to be rules for defaulting to attributes, including for when they are clearly not average (as indicated by damage bonus, HP or WP)