Source Amplitude: fix data region keyerror

[roman-yermilov-gl]

What

• The issue: https://github.com/airbytehq/oncall/issues/2525
• The problem: As data_region is not a requierd field we need to be sure the key exists, so we must use .get(<key>, "default") construction instead of config[key]

How

• Take value from a dictionary in a safe way
• Add some comments

[github-actions]

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[github-actions]

source-amplitude test report (commit 7bc744ddd7) - ✅

⏲️ Total pipeline duration: 14mn56s

Step	Result
Connector package install	✅
Build source-amplitude docker image for platform linux/x86_64	✅
Unit tests	✅
Acceptance tests	✅
Code format checks	✅
Validate airbyte-integrations/connectors/source-amplitude/metadata.yaml	✅
Connector version semver check	✅
Connector version increment check	✅
QA checks	✅

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[lazebnyi]

Need update doc, after that good to go.

[lazebnyi]

Add info about default value to doc