refactor(graph): init keyStrategyMap via copy constructor instead of stream API

[chrisis58]

Describe what this PR does / why we need it

Optimizes the initialization of the keyStrategyMap field during graph compilation to improve code readability and performance.

Does this pull request fix one issue?

NONE

Describe how you did it

Refactored the keyStrategyMap initialization to use the LinkedHashMap copy constructor instead of the Stream API.

Describe how to verify it

Added a new test case for NullKeyStrategy to verify the behavior.

Special notes for reviews

Behavior Changes:

• Previously, null values caused a "fail-fast" NullPointerException. The new implementation accepts null values (logic will fallback to ReplaceStrategy when encountering nulls).
• The map is now initialized as a LinkedHashMap, which preserves the iteration order of the source map. Previously, the order was not guaranteed.

[CLAassistant]

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