Walk items schema instead of walking instance data

[justin-tay]

Closes #989

This changes the behavior of the walk for the item walk listener to walk the schema instead of walking the instance data. All the relevant item schemas will be called at least once with null as the instance node (This is not the same as NullNode which is a node with a value of null while null is the absence of the node) when there is no applicable instance data.

This also fixes the incorrect schemaLocation and evaluationPath used for the tuple schemas for prefixItems.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.86957% with 63 lines in your changes are missing coverage. Please review.

Project coverage is 78.73%. Comparing base (3416e28) to head (05e5fd0).

Files	Patch %	Lines
...main/java/com/networknt/schema/ItemsValidator.java	66.26%	22 Missing and 6 partials ⚠️
...ava/com/networknt/schema/PrefixItemsValidator.java	48.14%	11 Missing and 3 partials ⚠️
...java/com/networknt/schema/DynamicRefValidator.java	0.00%	11 Missing ⚠️
...ava/com/networknt/schema/ItemsValidator202012.java	30.00%	4 Missing and 3 partials ⚠️
...va/com/networknt/schema/RecursiveRefValidator.java	81.81%	0 Missing and 2 partials ⚠️
...java/com/networknt/schema/PropertiesValidator.java	80.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #993      +/-   ##
============================================
- Coverage     78.81%   78.73%   -0.09%     
- Complexity     1927     1955      +28     
============================================
  Files           172      172              
  Lines          6213     6334     +121     
  Branches       1205     1247      +42     
============================================
+ Hits           4897     4987      +90     
- Misses          861      881      +20     
- Partials        455      466      +11     

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