There is no difference in making plasmid minipreps of empty vectors and in making plasmid minipreps of vectors with inserts. You just need to know what antibiotic resistance gene is carried by the vector, so when you grow your bacterial culture, you select for cells that contain the plasmid (empty or not).
However, I’m a bit confused about what your starting material is. Do your BL21 cells already contain the pET vector? If so they are no longer competent, they are just frozen cells containing plasmid. It is best to plate a loopfull of the frozen material on selective agar for single colonies first, then pick a nice healthy- looking, well separated colony, and use it as inoculum for the next step. And then you would grow them in the appropriate antibiotic broth and do the miniprep according to instructions.
However, If you have been given purified pET PLASMID DNA (not in cells) and you need to make more by putting it into BL21 competent cells first, you need to perform a step called transformation to get the plasmid into the competent cells. You will need to look up the procedure for transforming that strain of E. coli. If you have frozen COMPETENT cells, there is a procedure for thawing an aliquot, adding the plasmid DNA, then doing a treatment to make the plasmid enter the cells (usually a heat shock, unless you are using “electrocompetent” cells and electroporation), then after following all the instructions, you plate the cells on agar containing your selective antibiotic and incubate them. Next day look for surviving colonies and they should now contain the plasmid. Then you can pick a colony and grow it up in broth to make the plasmid prep.
Note: you generally need only a few nanograms of plasmid DNA to transform cells (lots less than you would use if you were transforming a LIGATION mixture.)
If your frozen BL21 cell stocks are just frozen cells WITHOUT vector, and not competent, then there is a procedure for making them competent. But for most people these days, it is a lot easier to buy stocks of frozen competent cells.
Hope this makes sense to you! Good luck!
Edited by OldCloner, 23 July 2019 - 11:21 AM.