Make linkedItems optional

[Simply007]

Motivation

Relates to #106.

Changes in #106 lead to a breaking change. Adding a parameter to the interface is a breaking change - to prevent this, it should be OK to make it optional.

Checklist

• [ ] Code follows coding conventions held in this repo
• [ ] Automated tests have been added
• [ ] Tests are passing
• [ ] Docs have been updated (if applicable)
• [ ] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

If manual testing is required, what are the steps?

[codecov[bot]]

Codecov Report

Merging #107 (dfc85fe) into master (22e2083) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #107   +/-   ##
=========================================
  Coverage     98.13%   98.13%           
  Complexity      195      195           
=========================================
  Files            22       22           
  Lines           591      591           
=========================================
  Hits            580      580           
  Misses           11       11           

Impacted Files	Coverage Δ
src/Kentico/Kontent/Delivery/DefaultMapper.php	98.46% <100.00%> (ø)

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[Simply007]

Before closing - fix wiki page about the resolver

[Simply007]

Let's keep it this way.