Abbreviate all-zero INIT macros as `{0}`?

[kainino0x]

Many of the INIT values (for base/standalone structs) will actually be all-zero, so equivalent to zero-initialization {}.
For those we could reduce the size of the header by just generating {} instead.

INIT structs are issue #158, PR #422

[eliemichel]

It seems that {} is not valid in C (it is reported by CI to be a non-standard GNU extension), we should rather use {0}.

[kainino0x]

Discussed on #427:

• Should we have INIT macros when they are equivalent to zero-init, and therefore not really necessary? (see also "Abbreviate all-zero INIT macros as {0}? #423")
  • Keep for same reason
  • CF: Think we should not abbreviate. They're a form of documentation.