fix(apply_grants): allow blank grant_config

[bassrock]

Description

This PR fixes apply_grants to ensure that grants_config is defined even if the user does not set it.

This aims to solve the following issue:

23:39:56  
23:39:56
23:39:56    Compilation Error in model snowplow_unified_app_errors (models/optional_modules/app_errors/snowplow_unified_app_errors.sql)
  'grant_config' is undefined

  > in macro apply_grants (macros/adapters/apply_grants.sql)
  > called by macro materialization_clone_default (macros/materializations/models/clone/clone.sql)
  > called by model snowplow_unified_app_errors (models/optional_modules/app_errors/snowplow_unified_app_errors.sql)

What type of PR is this? (check all applicable)

• [ ] 🍕 Feature
• [x] 🐛 Bug Fix
• [ ] 📝 Documentation Update
• [ ] 🎨 Style
• [ ] 🧑‍💻 Code Refactor
• [ ] 🔥 Performance Improvements
• [ ] ✅ Test
• [ ] 🤖 Build
• [ ] 🔁 CI
• [ ] 📦 Chore (Release)
• [ ] ⏩ Revert

Checklist

• [ ] 💣 Is your change a breaking change?
• [ ] 📖 I have updated the CHANGELOG.md

Added tests?

• [ ] 👍 yes
• [ ] 🙅 no, because they aren't needed
• [x] 🙋 no, because I need help

Added to documentation?

• [ ] 📓 internal package docs (ymls, macros, readme, if applicable)
• [ ] 📕 I have raised a Snowplow documentation PR if applicable (Link here if required)
• [x] 🙅 no documentation needed

[optional] Are there any post-deployment tasks we need to perform?

[optional] What gif best describes this PR or how it makes you feel?