[yaml2obj] - Make DynamicSymbols to be Optional<> too.

We already have Symbols property to list regular symbols and
it is currently Optional<>. This patch makes DynamicSymbols to be optional
too. With this there is no need to define a dummy symbol anymore to trigger
creation of the .dynsym and it is now possible to define an empty .dynsym using
just the following line:

DynamicSymbols: []
(it is important to have when you do not want to have dynamic symbols,
but want to have a .dynsym)

Now the code is consistent and it helped to fix a bug: previously we
did not report an error when both Content/Size and an empty
Symbols/DynamicSymbols list were specified.

Differential revision: https://reviews.llvm.org/D70956