fix(config): remove runtimeConfig string fallback

[kevinmarrec]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Follows #7420

Since we check for string type in interpolate function, there is no need to fallback to empty string value ''.

It fixes the issue where false was transformed to ''.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-commenter]

Codecov Report

Merging #7421 into dev will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #7421   +/-   ##
=======================================
  Coverage   70.20%   70.20%           
=======================================
  Files          88       88           
  Lines        3679     3679           
  Branches     1007     1007           
=======================================
  Hits         2583     2583           
  Misses        889      889           
  Partials      207      207           

Flag	Coverage Δ
#unittests	70.20% <100.00%> (ø)

Impacted Files	Coverage Δ
packages/config/src/load.js	76.82% <100.00%> (ø)

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