Is it "risky" to leave my MacBook Pro to Apple employees ? Should I take precautionary measures before going there ?
It doesn't matter they're Apple employees or any kind. Some people may take a look and steal, some not, no matter where they're working at.
This being said, unless you can get them (under whatever reasons) give your HDD (SSD?) back until they finish their job (drive staying always at your sight), it's safer you just get from start your funds out to another (safely generated) wallet and consider the old wallet compromised, just in case...
back up your Electrum wallet and then delete it from the MacBook before taking it to the Apple Store.
Well, he should wipe the file (and maybe the free space too), not just delete.
That's why moving the funds out is a safer approach.