Fix eslint warnimg

[lumburr]

What: fix #3224

Why:

How:

Use a new utils function replace v == null to fix eslint warning.
It's a code style issue. We also can chage ESlint config option "no-eq-null" to fix it. Which plan are we going to take? I will update my pr if it needs to change.

Checklist:

• Documentation N/A
• Added/updated unit tests N/A
• Code complete

[matthew-dean]

So, I don't agree with this approach. I think the better way is to figure out what foo == null was intended to check for. If it's just checking for a "falsey" value, it should just be !foo.

That is, someone should log all these cases of == null with a typeof during testing to see what exactly are the types of values getting loosely checked against null, and then re-write this in a way that is more clear. That's IMO a better approach rather than just adding a bunch of function wrappers, which possibly degrades performance.

[lumburr]

1. Conclusion: currentDirectory---- is related to the file, there are only two possible undefined, normal value
   currentDirectory comes from the file information in the node and will be given a default value of {}.
   Path: packages/less/src/less/environment/environment.js
   
   Path: packages/less/src/ess/tree/node.js
   
2. Conclusion: v has three possible values null, undefined, normal.
   Path: packages/less/src/less/functions/default.js
   
3. Conclusion: unit has only two possible values null, normal value.
   Path: packages/less/src/less/functions/math-helper.js
   
   MathHelper is only used in two places.
   Path: packages/less/src/less/functions/math.js
   
   It is normal.
   Path: packages/less/src/less/functions/number.js
   
   It is normal or null.
4. Conclusion: value.value has only two possible values undefined, normal value.
   Path: packages/less/src/less/parser/paser.js
   
   Path: packages/less/src/less/parser/paser.js
   
   It is normal or undefind.
   Path: packages/less/src/less/parser/paser.js
   
   It is undefind.
   Path: packages/less/src/less/parser/paser.js
   
   It is undefind.
   Path: packages/less/src/less/parser/paser.js
   
   It is normal or undefind.
5. Conclusion: this.visibilityBlocks has only two possible values undefined, normal value
   Path: packages/less/src/less/tree/node.js
   
   Default value is undefind, Invalid information is filtered.
6. Conclusion: escaped has two possible values, boolean value, default value undefined
   Path: packages/less/src/less/tree/quoted.js
   
   Path: packages/less/src/less/parser/parser.js
   
   It is undefind.
   Path: packages/less/src/less/parser/parser.js
   
   It is undefind or boolean.
7. Conclusion: nestedSelector has only two possible values null, normal value.
   Path: packages/less/src/less/tree/ruleset.js
   
   Path: packages/less/src/less/tree/ruleset.js
   
   nestedSelector instanceof Selector or return null.
8. Conclusion: evaldCondition has three possible values null, undefined, normal.
   Path: packages/less/src/less/tree/selector.js
   
   Path: packages/less/src/less/tree/ruleset.js
   
   It is undefind.
   Path: packages/less/src/less/tree/atrule.js
   
   It is null.

[matthew-dean]

Thanks for doing this research!