What does "-" in Google Adword's suggested bid mean?

It means there is not enough data over the last year for Google to determine that information. That could be for a number of reasons.

It could be low search volume (not in this case as you said). It could be a newly coined term or term with new popularity that does not have sufficient history, It could be that it is not a buyers keyword or keyword that would produce an action that an advertiser would find valuable. It could be that it falls under a category that is forbidden or highly restricted by Adwords or not allowed in your country. There are a number of other possibilities, including the one you are hoping for - that you've discovered an untapped niche/keyword. Keep in mind that that is the least likely of all the scenarios, but of course it can happen, so good luck.

The easiest way to see if you will have any competitors for your bid would be to type in the keyword in Google search and see if any ads come up. If you see any ads at all then you know you will not be able to just bid the minimum.