If you think about it, Genesis block and its reward (worth $1.5 million today) is technically a premine IF it could be spent and that's would not have been a good thing to have in Bitcoin.
I'm not sure that would have been a motivating factor for Satoshi. Was a "pre-mine" even a concept which existed prior to altcoin creators using it to make themselves richer at the expense of their users? Would Satoshi even have known of the concept of "pre-mining"? If he was overly concerned about being seen to be pre-mining, then it doesn't make sense for him to have mined the first block 5 days prior to announcing the release of the software to the mailing list.
And you could equally argue that the genesis block isn't a pre-mine; it's a regular mine. Pre-mining is setting aside x amount of coins before your chain is even launched. The genesis block wasn't that, but rather the standard block reward for mining a block. It just so happened to be the first block.
I don't read RIPEMD specification or how it works in detail, but did you mean RIPEMD-160 output always higher than zero?
RIPEMD-160, as the name suggests, always outputs a 160 bit number. If it was to output 0000000000000000000000000000000000000000, then that would give the address 1111111111111111111114oLvT2 as has been discussed above, which is not provably unspendable however unlikely it is that someone knows one of the private keys. However, because it always outputs a 160 bit number, it will never output 0, and so there is no private key which will be able to unlock this script and move these coins.