In the case, bitcoin transactions are becoming less frequent, will the blockchain become unsafe? The security of the blockchain relies in the fact that new blocks are added faster than any attacker guy can add them (as long as &lt;51%)

In case the frequency of the bitcoin transactions becomes less, an attacker could use the time to remove some previous transactions (not too far behind) and do bis proof of work.
Then... When new Block is finally verified and ready to be added, in the case he is the fastest miner (even &lt;51%), he could propagate his bogus blockchain fork.

Of course, due to &lt;51% his chances are little but because of the low amount of transactions, he could try over and over again (thus increasing his chances).

World this render blockchain more insecure? 

Thanks!
It's not the number of transactions that determines the security level, it's the amount of work done to partially invert the hash function. For the same difficulty level, this is proportional to the number of blocks mined (though Bitcoin scales the difficulty to even out the time between blocks). In the short run, the miners do not really care if there are many transactions or few transactions available to include in the block -- they've already invested in the high fixed cost of ASIC mining hardware.

To pull off a 51% attack, the attacker would have to provide as much computing power as all legitimate miners do. A slight drop in the popularity of Bitcoin isn't going to cut it.