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

[SarahFrench]

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"

---

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

[SarahFrench]

I'm not sure about how to word the change in the release note in my PR description - I'd appreciate any edits people see fit!

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 3 files changed, 128 insertions(+), 60 deletions(-))
Terraform Beta: Diff ( 3 files changed, 128 insertions(+), 60 deletions(-))
TF Conversion: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2564
Passed tests 2286
Skipped tests: 275
Affected tests: 3

Action taken

Found 3 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample|TestAccApigeeKeystoresAliasesPkcs12_ApigeeKeystoresAliasesPkcs12Example|TestAccDataSourceGoogleFirebaseAndroidAppConfig

Get to know how VCR tests work

[modular-magician]

Tests passed during RECORDING mode:
TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample[Debug log]
TestAccApigeeKeystoresAliasesPkcs12_ApigeeKeystoresAliasesPkcs12Example[Debug log]

Tests failed during RECORDING mode:
TestAccDataSourceGoogleFirebaseAndroidAppConfig[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test

[SarahFrench]

Saving any testing of Application Default Credentials for a separate PR to this one- merging now