Fix NPE in CodegenIgnoreProcessor.java (fixes #3356) (#7846)

Fixes NullPointerException when using an override ignore file with no parent.

Author: jbrower2

modules/openapi-generator/src/main/java/org/openapitools/codegen/ignore/CodegenIgnoreProcessor.java

@@ -142,7 +142,7 @@ void loadCodegenRules(final File codegenIgnore) throws IOException {
     public boolean allowsFile(final File targetFile) {
         if(this.ignoreFile == null) return true;
 
-        File file = new File(this.ignoreFile.getParentFile().toURI().relativize(targetFile.toURI()).getPath());
+        File file = new File(this.ignoreFile.getAbsoluteFile().getParentFile().toURI().relativize(targetFile.toURI()).getPath());
         Boolean directoryExcluded = false;
         Boolean exclude = false;
         if(exclusionRules.size() == 0 && inclusionRules.size() == 0) {