Missing template values are silently rendered as `<no value>`

[dadrus]

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Describe the bug

When a Go template references a value that is not available in its evaluation context, it can silently render as <no value> instead of failing.

For endpoint URLs or headers this may result in a syntactically valid but incorrect downstream request. The resulting 400/401 response then hides the actual configuration problem.

See #3495 for the context in which this was discovered.

Expected behavior: Missing or unavailable template values should cause rendering to fail with an actionable template error instead of silently producing <no value>.

How can the bug be reproduced

Configure an endpoint template with an unavailable value, e.g.:

endpoint:
  url: http://api.internal/entitlements?tenantId={{ .Request.URL.Captures.tenantId }}

Execute the mechanism.

The rendered URL contains:

tenantId=<no value>

and the request is sent downstream instead of failing during template rendering.

Relevant log output

DBG Creating request _endpoint="http://api.internal/entitlements?tenantId=<no value>"
INF Pipeline step execution failed error="authorization error: authorization failed based on received response code: 400"

Relevant configuration

See reproduction above and #3495.

Version

v0.17.21

On which operating system are you observing this issue?

Linux

In which environment are you deploying?

Docker

Additional Context

Originally identified while investigating #3495.