Use default send after for empty string

[trodge]

fixes hashicorp/terraform-provider-google#14250

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• 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)

all: fixed an error resulting from leaving `provider.batching.send_after` unspecified and `provider.batching` specified

[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 ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
TF Conversion: Diff ( 3 files changed, 4 insertions(+), 4 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]
TestAccDataSourceGoogleFirebaseAndroidAppConfig[Debug log]

All tests passed
View the build log or the debug log for each test