[yaml2obj] - Refine handling of the NoHeaders key.

Imagine we have an YAML description for some object and we want to
produce 2 outputs: with and without the section header.
A natural way to do it would look like:

```
--- !ELF
FileHeader:
  Class:   ELFCLASS64
  Data:    ELFDATA2LSB
  Type:    ET_REL
  Machine: EM_X86_64
Sections:
...
SectionHeaderTable:
  NoHeaders: [[NOHEADERS]]

```
But currently, we do not distinguish between no `NoHeaders` key case
and `NoHeaders == false`. Because of this we can't simply specify
`NOHEADERS = false`, as tool starts to complain.

With this patch the behavior changed. When we have:

```
SectionHeaderTable:
  NoHeaders: false

```
it is the same as we have no `SectionHeaderTable` at all.
(`NoHeaders` key still can't be used with `Sections/Excluded` keys)

Differential revision: https://reviews.llvm.org/D83672