`__lib_either_version_flag` returning `None` triggers `NameError`

I call `SetAllowableExceptions()` in my build files to ensure that I set
all applicable build variables for my targets. I then set variables I don't
currently need to the empty string, to avoid the resulting `NameError`s.

After a recent update to Scons 4.7.0, I was getting a `NameError` stating
that `__libversionflags` was undefined. This variable is implemented as a
function, and the docstring states: `if version_var is not empty, returns
env[flags_var], otherwise returns None`.

https://github.com/SCons/scons/blob/5dd1d9cea01dd639604b6a4cad911ff8437bd961/SCons/Defaults.py#L683

`__libversionflags` is used in `__SHLIBVERSIONFLAGS` and other variables:

```
'__SHLIBVERSIONFLAGS':
'${__libversionflags(__env__,"SHLIBVERSION","_SHLIBVERSIONFLAGS")}'
```

I do not require versioned shared libraries for this particular build, so I
have set ` SHLIBVERSION` to the empty string, which does not trigger a
`NameError`. Unfortunately, this causes `__libversionflags` to return
`None`, which is interpreted as an undefined variable, thus raising a
`NameError`:

https://github.com/SCons/scons/blob/5dd1d9cea01dd639604b6a4cad911ff8437bd961/SCons/Subst.py#L394

I propose that `__libversionflags` be modified to return the empty string
if `version_var` is empty. Note that a similar issue exists for
`__lib_either_version_flag` in the same file; I propose the same change
here too.

Thank you,
Sye van der Veen

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