Why frequency of a DC signal is chosen as zero? If the period can be anything, isn't it same for frequency?
If you want to treat this signal as a periodic one, then you can take its Fourier series.

Unlike most other periodic signals, you have free choice of what frequency to consider as its fundamental frequency --- you can calculate the series for any fundamental frequency

But regardless of which fundamental frequency you choose, you'll find that all the terms in the series except for the 0-frequency one have zero magnitude.