A hash is a binary string but since Nix files are plain text, using the hash as is would not be very convenient. For that reason a hash is typically represented as a number. To make the number shorter bases other than the common base-10 is used. Nix supports the following, demonstrated on SHA-256 hash with the numerical value 0:
- base-64: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
- base-32: 0000000000000000000000000000000000000000000000000000
- base-16: 0000000000000000000000000000000000000000000000000000000000000000
Nix uses the length of the hash to determine the format. Since you replaced the base-64 hash with zeroes it was considered a base-64 format. In the encoding, each digit represents 6 bits so you need ⌈256/6⌉=43 characters. Since RFC 4648 wants the base-64 numbers to have lengths divisible by four, = is required as a padding. But you cannot really change it to 0, as that would give you 44*6=264 bits.