You're going to come across this sentiment alot because,many people in the jazz world would rather hear,and see the upright.
The good news is,everyone in the jazz world doesn't hold that sentiment. If you know the music,have a good,unobtrusive tone ,and can swing, then you're good to go!
I've been in this business for 40 years and counting,and I can tell you that 80% of my gigs are straight-ahead jazz dates,...and I play electric bass,exclusively! So,don't be discouraged!
Going to sit in at the jazz clubs is a great idea,and can be very beneficial for you,in the long run. You first have to learn some tunes in order to get yourself started.
Secondly,if you haven't already done so,it would be in your best interest and much to your advantage to attend some form of academia(music school,jazz workshop,private instructor)so you can learn the language.
Thirdly,don't worry about the fact that,you don't own an upright! Start learning on your electric bass so that you can learn the music,first! Then,if you want to get yourself an upright,then by all means,...go for it!
Let me share this with you,in closing,and this is something that I posted,previously.
When I was a younger musician,I was playing a gig with the Gillie Coggins Trio at a place called the "Village Door" here in New York,and in between sets I was having a conversation with some cats about music!
Long story short, one of these people said to me:"You can't truly play jazz,unless you play upright!"
My reply to him was:"I don't care if you play upright,electric,or a rubber-band wrapped around a shoebox.If you can't truly play jazz,it's because you don't know the music!"
This goes for any other genre,as well!
I've said all of that to say,music begins with the MUSICIAN,....period! The instrument is the vehicle by which,this gift gets conveyed.
So,don't listen to the nay-sayers,....if this is something you TRULY want to do,then go for it! By ALL means!
Peace!