Allow `None` in `LinkerInfo.linker_flags`.

[zadlg]

Allow None in LinkerInfo.linker_flags.

LinkerInfo.linker_flags field is by default set to None, as well as other flag list fields (e.g. LinkerInfo.archiver_flags).

However, LinkerInfo.linker_flags isn't consistenly used across prelude.

For instance, in cxx/cxx_library.bzl, LinkerInfo.linker_flags is treated as an optional list:

https://github.com/facebook/buck2/blob/1e89af6622344c46543a9e3781a556e058471043/prelude/cxx/cxx_library.bzl#L774

On the other hand, in cxx/cxx_toolchain_types.bzl, LinkerInfo.linker_flags is expected to contain a non-None value:

https://github.com/facebook/buck2/blob/1e89af6622344c46543a9e3781a556e058471043/prelude/cxx/cxx_toolchain_types.bzl#L308

https://github.com/facebook/buck2/blob/1e89af6622344c46543a9e3781a556e058471043/prelude/cxx/cxx_toolchain_types.bzl#L327

This leads to an error when defining a custom CXX toolchain:

Caused by:
    0: Error running analysis for `toolchains//:cxx (prelude//platforms:default#904931f735703749) (prelude//platforms:default#904931f735703749)`
    1: Traceback (most recent call last):
         File <builtin>, in <module>
         * musl-toolchain/define_toolchain.bzl:53, in _define_cxx_toolchain_impl
             ] + cxx_toolchain_infos(
         * prelude/cxx/cxx_toolchain_types.bzl:308, in cxx_toolchain_infos
             "ldflags-shared": _shell_quote(linker_info.linker_flags),
         * prelude/cxx/cxx_toolchain_types.bzl:328, in _shell_quote
             return cmd_args(xs, quote = "shell")
       error: Expected `a value implementing CommandLineArgLike (str, Artifact, RunInfo, etc)`, but got `NoneType`
          --> prelude/cxx/cxx_toolchain_types.bzl:328:12
           |
       328 |     return cmd_args(xs, quote = "shell")
           |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
           |

This commit handles the LinkerInfo.linker_flags consistently across the project.

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