NNS1-3094: Show -/- instead of 0 when neuron count not available

[dskloetd]

Motivation

To make it clear that the neuron count is not 0 when the user is not signed in or the neurons haven't loaded yet.

Changes

1. Pass neuronsStore instead of definedNeuronsStore to be able to tell if NNS neurons were loaded.
2. Filter NNS neurons without valid stake as was previously done in definedNeuronsStore.
3. Use undefined when the user is not signed in, or neurons haven't loaded, to indicate that neuron count is unavailable.
4. Render undefined as -/- in the neurons cell.

Tests

1. Unit tests added.
2. Tested manually at https://qsgjb-riaaa-aaaaa-aaaga-cai.dskloet-ingress.devenv.dfinity.network/staking/

Todos

• Add entry to changelog (if necessary).
  not yet

[mstrasinskis]

Thanks!