Fix lazy field null handling and clone preservation

[dereuromark]

Summary

• Use array_key_exists() instead of isset() in method_get.twig template to correctly detect null values in lazy data (isset() returns false for null values, causing lazy null values to be ignored)
• Copy _lazyData in clone() method to preserve unhydrated lazy data when cloning DTOs
• Add tests for clone with lazy data and null lazy value handling

Issues Found

1. Getter template uses isset() for lazy data check

// Before (wrong) - isset() returns false for null values
if (isset($this->_lazyData['field'])) {

// After (correct) - array_key_exists detects the key regardless of value
if (array_key_exists('field', $this->_lazyData)) {

2. clone() method doesn't copy _lazyData
When cloning a DTO with unhydrated lazy fields, the lazy data was not preserved in the clone, causing the clone to lose access to that data.

Relates to #83

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.47%. Comparing base (7f084c5) to head (fab274a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #89   +/-   ##
=========================================
  Coverage     83.46%   83.47%           
  Complexity     1386     1386           
=========================================
  Files            40       40           
  Lines          3345     3346    +1     
=========================================
+ Hits           2792     2793    +1     
  Misses          553      553           

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