Fix comparison with nothing to use idetity checking

[Paalon]

Comparison with nothing should use identity checking operators like === or !==.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.23%. Comparing base (844ad27) to head (d69529b).
Report is 5 commits behind head on master.

Files	Patch %	Lines
src/composer.jl	0.00%	1 Missing ⚠️
src/constructor.jl	0.00%	1 Missing ⚠️
src/parser.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   86.59%   86.23%   -0.37%     
==========================================
  Files          12       12              
  Lines        1626     1627       +1     
==========================================
- Hits         1408     1403       -5     
- Misses        218      224       +6     

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[Paalon]

If we can drop support for Julia v1.0, we should use isnothing for comparing nothing.

[kescobo]

According to this, these are equivalent? But I think this is more idiomatic