Prevent a crash when using --repo_env=VAR without a value

The --repo_env option is documented in the same way as --action_env. In
addition to allowing you to set explicit values, you can also use it to
explicitly pick values from the environment in which Bazel was invoked.
Unfortunately, this causes a null pointer exception in Starlark due to a
null string stored as a map value.

This change extends the logic of converting --repo_env to a map to take
null values into account. When null, the value is loaded from the
current environment. This behaviour is useful in case you want to do
something like this:

--incompatible_strict_action_env --action_env=PATH=... --repo_env=PATH

This allows you to run build actions with a strict value for $PATH (e.g.,
to get a decent action cache hit rate in case of remote execution),
while still allowing repository_ctx.which() to find tools on the host
system using $PATH.

Closes #12886.

PiperOrigin-RevId: 362506900