Exists method for check if Any object is empty

[mmaryo]

Add method to check if a field exists

Before :

            Any rowField = atFields.get(mappingField.getAirtableFieldName());
            boolean exists = true;
            try {rowField.mustBeValid();}catch(Exception e){exists = false;}
            if (exists) {
                
            }

Now :

            Any rowField = atFields.get(mappingField.getAirtableFieldName());
            if (rowField.exists()) {
                
            }

[codecov-io]

Codecov Report

Merging #230 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
- Coverage   68.46%   68.45%   -0.01%     
==========================================
  Files         107      107              
  Lines        7335     7336       +1     
  Branches     1388     1389       +1     
==========================================
  Hits         5022     5022              
- Misses       1867     1868       +1     
  Partials      446      446

Impacted Files	Coverage Δ
src/main/java/com/jsoniter/any/Any.java	59.13% <0%> (-0.52%)	⬇️

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