allow NULL as name for iso_format to suppress selective exclusion of name

[sebkopf]

omit names for all parameters that are named NULL,

e.g. iso_format(NULL = "test", b = 42, NULL = 5) currently makes "NULL: test\nb: 42\nNULL: 5" when it should instead make "test\nb: 42\n5" by omitting the names of the parameters named NULL.