CIF-2385: fix NPE in ProductListItemImpl#generateId for placeholder data

[buuhuu]

Description

Removing the skus from the placeholder data causes some venia integration tests to fail with this NPE.

Related Issue

CIF-2385
CIF-2334 (cause)

Motivation and Context

How Has This Been Tested?

unit tests

Screenshots (if appropriate):

Types of changes

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

Checklist:

• I have signed the Adobe Open Source CLA.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes and the overall coverage did not decrease.
• All unit tests pass on CircleCi.
• I ran all tests locally and they pass.

[codecov]

Codecov Report

Merging #677 (489016e) into master (460a61f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #677   +/-   ##
=========================================
  Coverage     88.58%   88.58%           
  Complexity     1598     1598           
=========================================
  Files           282      282           
  Lines          7129     7129           
  Branches       1046     1046           
=========================================
  Hits           6315     6315           
  Misses          618      618           
  Partials        196      196           

Flag	Coverage Δ
integration	59.74% <100.00%> (ø)
jest	85.61% <ø> (ø)
karma	90.01% <ø> (ø)
unittests	89.04% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...internal/models/v1/common/ProductListItemImpl.java	96.92% <100.00%> (ø)

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