fix(template-strings): do not apply helper functions on unresolved string

[shumailxyz]

What this PR does / why we need it:
Before this fix, helper functions were being applied even on resolved string and the unsubstituted string was considered the string itself for helper function.

Basically, in the first resolveTemplateStrings of module, when only inputs are available and keys like environment are missing, unresolved strings are fine as allowPartial is set to true. However, the helper functions were still being applied.

For example, for a template string like

TEST:  "base64Encode(\"${environment.namespace}\")"`

base64Encode was being called for environment.namespace instead of the actual value and resulted in wrong values. So before the 2nd resolveTemplateStrings of module, when the module context is available, there was no base64Encode(\"${environment.namespace}\") left to be resolved.

The workaround mentioned in original issue #4614
TEST: "${environment && base64Encode(\"${environment.namespace}\")}"
works because of the environment &&. Even though the base64Encode is applied on environment.namespace, the overall expression fails due to environment missing in first pass.

I added a check before helper functions are applied to not apply helper on unresolved string. Had to wrap text in ${} so it's returned as an expression to be resolved later. In case, allowPartial is set to false, it returns error.

Which issue(s) this PR fixes:

Fixes #4614

Special notes for your reviewer:
I am not sure if we can avoid this check altogether and instead extend the actual parser to address this issue. 🤔

[vvagaytsev]

LGTM, but I'd like @edvald to take a look too. Can we add some test cases? The repro examples from the original issue can be used as a base.

[edvald]

Fix looks great (and very well caught @shumailxyz!) but agree that a test case would be helpful.