Avoid adding a token at NULL_ADDRESS

[pirapira]

because then the call to TokenNetworkRegistry will fail.

This fixes #4875.

Fixes: 4815

Description

Please, describe what this PR does in detail:

• If it's a bug fix, detail the root cause of the bug and how this PR fixes it.

After this PR, TokenNetworkRegistry proxy refuses to register a token contract at address 0x00000000....00. The constructor of TokenNetwork contract checks against this case, so the contract proxy needs to check this condition.

I didn't add post-failure checks because the precondition checks will completely remove the possibility that the token address is 0x00000000..00.

PR review check list

Quality check list that cannot be automatically verified.

• Safety
  • The changes respect the necessary conditions for safety (https://raiden-network-specification.readthedocs.io/en/latest/smart_contracts.html#protocol-values-constraints)
• Code quality
  • Error conditions are handled
  • Exceptions are propagated to the correct parent greenlet
  • Exceptions are correctly classified as recoverable or unrecoverable
• Compatibility
  • State changes are forward compatible
  • Transport messages are backwards and forward compatible
• Commits
  • Have good messages
  • Squashed unecessary commits
• If it's a bug fix:
  • Regression test for the bug
    • Properly covers the bug
    • If an integration test is used, it could not be written as a unit test
• Documentation
  • A new CLI flag was introduced, is there documentation that explains usage?
• Specs
  • If this is a protocol change, are the specs updated accordingly? If so, please link PR from the specs repo.
• Is it a user facing feature/bug fix?
  • Changelog entry has been added

[codecov]

Codecov Report

Merging #4876 into develop will decrease coverage by 0.16%.
The diff coverage is 75%.

@@             Coverage Diff             @@
##           develop    #4876      +/-   ##
===========================================
- Coverage    80.97%   80.81%   -0.17%     
===========================================
  Files          120      120              
  Lines        14510    14515       +5     
  Branches      2236     2238       +2     
===========================================
- Hits         11750    11730      -20     
- Misses        2112     2129      +17     
- Partials       648      656       +8

Impacted Files	Coverage Δ
raiden/api/rest.py	73.26% <ø> (ø)	⬆️
raiden/exceptions.py	100% <100%> (ø)	⬆️
raiden/network/proxies/token_network_registry.py	77.14% <100%> (+0.44%)	⬆️
raiden/api/python.py	68.08% <50%> (-0.9%)	⬇️
raiden/storage/serialization/types.py	64.17% <0%> (-4.48%)	⬇️
raiden/tasks.py	67.64% <0%> (-2.95%)	⬇️
raiden/utils/echo_node.py	67.79% <0%> (-1.7%)	⬇️
raiden/network/transport/matrix/client.py	68.5% <0%> (-1.3%)	⬇️
raiden/network/proxies/token_network.py	50.06% <0%> (-0.74%)	⬇️
raiden/raiden_event_handler.py	91.84% <0%> (-0.63%)	⬇️
... and 3 more

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Contributed to the PR

[LefterisJP]

Fixes: #4815

How does this relate to #4815?