DefaultTemplateConfigMaxConnsPerHost not enforced if template_config block is missing

[indjb]

Describe the bug
#24548 adds support for setting MaxConnectionsPerHost in a Vault-agent configuration, including a default of 10. But the default is not used if the template_config block is totally missing from the configuration.

To Reproduce
Run Vault agent using a configuration that includes no template_config block. Observe that connections are not limited and pooled as they should be.

Expected behavior
The default should be enforced even when a template_config block is missing.

Environment:
This bug exists on the main branch.