They only made 391,308,000 of them without a mintmark. Now...., if you would find one WITH a P mintmark, THAT would be newsworthy.
Hint: Same dealio with 1979. Jus' sayin'. The widespread use of the P mintmark began in 1980, which, incidentally, feels like three weeks ago to me. Best girlfriend I ever had. Shouldn't have let that one get away.
Not all nickels minted have Mint marks. Yours is one of them. It was made this way by the Mint. In 1978 the law did not allow for the Philadelphia Mint to put a P on the coin. It's normal so spend it. Get a Red Book by Yeoman. You can learn a lot from it. Welcome to CT.
Brief history of the P mintmark on US coins: 1. Before 1980 the ONLY US coins that had P mintmarks were the 35% silver warnickels from 1942 - 1945, and the 1979 SBA dollar. 2. 1980 and later ALL US coins struck in Philadelphia had a P mintmark except the cent, and the cent DOES have a P mintmark in 2017 only (so far). Your 1978 nickel falls under #1, the 1984 nickel falls under #2. Any questions?