fix(guide): typos and change truthy to falsy (#19064)

A variable declared without a value, will be undefined, therefore it is falsy, not truthy (the example could have been pasted badly). Also changed falsey to falsy and added '' as another way to declare an empty string.

client/src/pages/guide/english/javascript/falsy-values/index.md

@@ -4,7 +4,7 @@ title: Falsy Values
 
 ## Description
 
-A falsy value is something which evaluates to FALSE, for instance when checking a variable. There are only six falsey values in JavaScript: `undefined`, `null`, `NaN`, `0`, `""` (empty string), and `false` of course.
+A falsy value is something which evaluates to FALSE, for instance when checking a variable. There are only six falsy values in JavaScript: `undefined`, `null`, `NaN`, `0`, `""` or `''` (empty string), and `false` of course.
 
 ## Checking for falsy values on variables
 
@@ -25,7 +25,7 @@ var filledString = "some string in here"; // <-- truthy
 
 var zero = 0; // <-- falsy
 
-var numberGreaterThanZero // <-- truthy
+var numberGreaterThanZero; // <-- falsy
 
 var emptyArray = []; // <-- truthy, we'll explore more about this next