Handling Nulls in Currency Filter

[AnushaKaraka]

This PR is to add a quick null check for input in the Currency filter and added an associated testcase.

Without this check a null reference exception is thrown whenever the input is null(which is a quite common scenario if we pass in a property of a model) and stopping the render of the entire template.

PS: Tried to create an issue first but couldn't for some reason. hence opened the PR directly. Please let me know if I have to follow a different process for the same. Here is my issue notes:

Dotliquid version 2.1.457

Expected behavior To render null if the input is null for a currency filter

Actual behavior It is throwing a null reference exception if I pass in a null property(which is highly possible)

Steps to reproduce the Problem (you can add files)

1. Create a customer object with a property named Amount.
2. Instantiate the object with a null value for Amount
3. Use DotLiquid syntax with currency filter {{Customer.Amount | currency:'fr-CA'}}

or

Use DotLiquid syntax with currency filter {{null | currency:'fr-CA'}}

[codecov]

Codecov Report

Merging #477 (9dc6dc5) into master (c9aff49) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #477   +/-   ##
=======================================
  Coverage   87.27%   87.28%           
=======================================
  Files          67       67           
  Lines        2633     2634    +1     
  Branches      634      635    +1     
=======================================
+ Hits         2298     2299    +1     
  Misses        212      212           
  Partials      123      123           

Impacted Files	Coverage Δ
src/DotLiquid/StandardFilters.cs	93.62% <100.00%> (+0.01%)	⬆️

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[microalps]

Would like to see a test and handling for empty string once we are at it.