A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
I can't believe that the search function didn't answer this question (perhaps my criteria were wrong) so I'm doing it now.
Can the attacker play the Naval reaction if his army is unopposed?
Naval Ability
Reaction: If you are the Attacker and your current army has more Naval cards than the opposing army, once per battle after engaging: You have the first opportunity to take a Battle action, which must be performed by a target Naval Personality or one of his Naval Followers.
Can the attacker play the Naval reaction if his army is unopposed?
Yes, if his current army has at least one Naval card, because the number of Naval cards in the opposing "army" (with no units) is treated as zero (see below, emphasis mine).
Glossary, Army: This is similar to the rules for absent stats; if anything in the game needs to know the Force, number of units, or other numerical aspect of an "army" with no units, it is always zero.