fix: use array length properly

[mdelapenya]

What does this PR do?

It fixes how array lengths are calculated when retrieving the files to be uploaded to GC storage.

Intead of using .length() as a method, which is incorrect, it uses .length as an attribute. As an alternative we could use .size() method.

Why is it important?

We are seeing this failure in the pipeline:

groovy.lang.MissingMethodException: No signature of method: [Lorg.jenkinsci.plugins.pipeline.utility.steps.fs.FileWrapper;.length() is applicable for argument types: () values: []
Possible solutions: last(), init(), getAt(groovy.lang.EmptyRange), getAt(java.util.Collection), getAt(groovy.lang.Range), getAt(java.lang.Integer)

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Caused by chore: upload files to Google Storage when they exist #28836

[elasticmachine]

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Build stats

• Start Time: 2021-11-09T06:33:03.342+0000

• Duration: 101 min 39 sec

• Commit: a3ab819

Test stats 🧪

Test	Results
Failed	0
Passed	54160
Skipped	5345
Total	59505

💚 Flaky test report

Tests succeeded.

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