Remove nullptr safety check from sax_parse functions (#5139)

PR #4873 introduced a safety check in sax_parse functions to catch
nullptr passed as SAX parser object, which had been already annotated by
JSON_HEDLEY_NON_NULL macro.

Compilers (e.g. clang) which respected the non-null annotation tended to
eliminate the safety check completely in optimized builds, while
compilers which did not, compiled the safety check in. This led to
different behaviors accross different compilers/platforms  and/or build
types (debug, release).

This commit reverts PR #4873 to remove this discrepancy. Passing null to
non-null annotated parameter is considered to be undefined behavior.

Fixes #5048

Signed-off-by: Richard Musil <risa2000x@gmail.com>
Co-authored-by: Richard Musil <risa2000x@gmail.com>

docs/mkdocs/docs/home/exceptions.md

@@ -900,13 +900,3 @@ A JSON Patch operation 'test' failed. The unsuccessful operation is also printed
     ```
     [json.exception.other_error.501] unsuccessful: {"op":"test","path":"/baz","value":"bar"}
     ```
-
-### json.exception.other_error.502
-
-This exception is thrown when a null pointer is passed as SAX event listener to the [sax_parse](../api/basic_json/sax_parse.md) function.
-
-!!! failure "Example message"
-
-    ```
-    [json.exception.other_error.502] SAX handler must not be null
-    ```