create-testnet-data: don't fail trying to create irrelevant READMEs

[smelc]

Changelog

- description: |
    Avoid an internal crash of `create-testnet-data`
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - improvement    # QoL changes e.g. refactoring
  - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Fixes #587

When using create-testnet-data in IntersectMBO/cardano-node#5646, I was faced with the fact that it was crashing when omitting some flags, because it was trying to create the README for some keys, whose parent directory wasn't being created; because the concerned keys where not requested. This is #587.

When fixing that, I also noticed that create-testnet-data was looping when neither --stake-delegators nor --transient--stake-delegators was specified. This was due to a zipWith call with concat $ repeat ([]) as an argument (the crux is to pass [] to repeat). So this PR also fixes that.

On the way, I also noticed that create-staked and create-testnet-data were avoiding a division by zero because of laziness. I made this less brittle by avoiding the division by zero explicitly.

How to trust this PR

A test is introduced. The test fails before this PR and it succeeds after the PR.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff