sd-json: make sure all dispatch helpers do something sensible in case of "null" JSON...

sd-json: make sure all dispatch helpers do something sensible in case of "null" JSON value

Most of our dispatch helpers already do something useful in case they
are invoked on a null JSON value: they translate this to the appropriate
niche value for the type, if there is one.

Add the same for *all* dispatchers we have, to make this fully
systematic.

For various types it's not always clear which niche value to pick. I
opted for UINT{8,16,32,64}_MAX for the various unsigned integers, which
maps our own use in most cases. I opted for -1 for the various signed
integer types. For arrays/blobs of stuff I opted for the empty
array/blob, and for booleans I opted for false.

Of course, in various cases this is not going to be the right niche
value, but that's entirely fine, after all before a json value reaches a
dispatcher function it must pass one of two type checks first:

1. Either the .type field of sd_json_dispatch_field must be
   _SD_JSON_VARIANT_TYPE_INVALID to not do a type check at all

2. Or the .type field is set, but then the SD_JSON_NULLABLE flag must be
   set in .flags.

This means, accidentally generating the niche values on null is not
really likely.