Fix root classpath resolve

[justin-tay]

Fixes the IRI resolve at the root when there is no starting slash.

For instance given classpath:resource.json and resolving test.json will now return classpath:test.json.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 78.79%. Comparing base (7abb68c) to head (4e0bd48).

Files	Patch %	Lines
...rc/main/java/com/networknt/schema/AbsoluteIri.java	75.00%	1 Missing and 2 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1000      +/-   ##
============================================
+ Coverage     78.73%   78.79%   +0.06%     
- Complexity     1955     1959       +4     
============================================
  Files           172      172              
  Lines          6334     6344      +10     
  Branches       1247     1252       +5     
============================================
+ Hits           4987     4999      +12     
+ Misses          881      876       -5     
- Partials        466      469       +3     

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