[@foal/cli] Fix --register failure when no imports

[LoicPoullain]

Issue

Resolves #606: when the file app.controller.ts does have any imports, the command foal generate controller --register fails.

Solution and steps

• Fix the bug
• Improve code

Checklist

• Add/update/check docs (code comments and docs/ folder).
• Add/update/check tests.
• Update/check the cli generators.

[codecov]

Codecov Report

Merging #619 into v1-6-0 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           v1-6-0     #619   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files          90       90           
  Lines        1632     1632           
  Branches      363      364    +1     
=======================================
  Hits         1626     1626           
  Misses          6        6

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