Sprig_fail not correct work

Examples:

```go
{{ if eq (len nodes) 0 }}
{{ sprig_fail "ERROR" }}
{{ end }}

Result:
execute: template: :35:9: executing "" at : error calling sprig_fail: Error

{{ $test := nodes }}
{{ if $test }}
{{ sprig_fail }}
{{ end }}

Result:
execute: template: :35:9: executing "" at : error calling sprig_fail: Error

{{ $test := nodes }}
{{ if not $test }}
{{ sprig_fail }}
{{ end }}

Result:
execute: template: :35:9: executing "" at : error calling sprig_fail: Error
```

Unexpected behavior.

If any information is needed, I am ready to provide it.

consul-template v0.41.1 (80a4e25)

Start by reproducing the three template examples using consul-template v0.41.1 and compare the reported results. Locate the sprig_fail implementation and its existing tests, then clarify the expected behavior for each conditional case before adding a regression test and verifying the corrected error handling.

Written by the indexing model from the issue text.