fix: handle null start/end times in basic_rates - #3722

Summary

• dict.get("start", "00:00:00") returns None when the key exists with an explicit null value — the default only applies when the key is missing entirely
• Use or "00:00:00" to fall back for both missing and null start/end times in rate entries
• Also guards the return value of resolve_arg() which can return None in edge cases

Context

Customer on manual_v2 energy provider with a flat export rate — the tariff wizard saved "start": null, "end": null for the flat rate entry, causing:

AttributeError: 'NoneType' object has no attribute 'count'
  File "/addon/fetch.py", line 1554, in basic_rates
    if start_str.count(":") < 2:

Test plan

• Existing basic_rates tests still pass
• Flat rate with null start/end no longer crashes

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