Linen: Consider raising an error when reading variables from submodule before initialization?

Within a module that uses shape-inference (as most of the built-in Linen modules do), this code is fine:

```
class MyModule(nn.Module):
def __call__(self, x):
conv = nn.Conv(features=3)
y = conv(x)
params = conv.params()
```

But if you instead do:
```
class MyModule(nn.Module):
def __call__(self, x):
conv = nn.Conv(features=3)
params = conv.params()
y = conv(x)
```

Then I believe you get an empty `params` dict (as the parameters of `conv` are only initialized once the input shape is known)

Seems like users may be surprised about this, so instead we could just raise an error if the variables are empty when reading them clarifying what is happening, to guide users to the right direction.