Fix empty string credentials validation issue, increase test coverage of credential validation

[modular-magician]

Fixes hashicorp/terraform-provider-google#14255

This PR:

• Improves testing for credential validation in the SDK provider config code
• Makes the tests for the plugin framework provider's config code match the new SDK tests
• Changes the new credential validator to allow empty strings as values of credentials in the provider config

Here are details of old tests that I replaced with test cases in a table-driven test

Removed test	Equivalent test case in new test
TestProvider_loadCredentialsFromFile (SDK version)	TestProvider_validateCredentials's "configuring credentials as a path to a credentials JSON file is valid"
TestProvider_loadCredentialsFromJSON (SDK version)	TestProvider_validateCredentials's "configuring credentials as a credentials JSON string is valid"
TestFrameworkProvider_loadCredentialsFromFile	TestFrameworkProvider_CredentialsValidator's "configuring credentials as a path to a credentials JSON file is valid"
TestFrameworkProvider_loadCredentialsFromJSON	TestFrameworkProvider_CredentialsValidator's "configuring credentials as a credentials JSON string is valid"

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If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

provider: fixed bug where `credentials` field could not be set as an empty string

Derived from GoogleCloudPlatform/magic-modules#7690