Cover case of an empty string in get-string-permutations

When adding a test case for an empty string, the code was looping
endlessly.

Author: Luciano Medeiros Marcelino

src/_Problems_/get-string-permutations/get-string-permutations.test.js

@@ -22,6 +22,13 @@ describe('Get permutations of a string', () => {
     expect(getPermutations(shortString)).toEqual(expectedPermutations);
   });
 
+  it('returns an empty array for an empty string', () => {
+    const shortString = '';
+    const expectedPermutations = [];
+
+    expect(getPermutations(shortString)).toEqual(expectedPermutations);
+  });
+
   it('is case sensitive', () => {
     const shortString = 'aB';
 

src/_Problems_/get-string-permutations/index.js

@@ -1,15 +1,19 @@
 // GET PERMUTATION OF A GIVEN STRING
 
 const getPermutations = (str) => {
+  let result = [];
+
+  if (str.length == 0) {
+    return result;
+  }
+
   if (str.length == 1) {
-    let array = [];
-    array.push(str);
-    return array;
+    result.push(str);
+    return result;
   }
 
   let currentCharacter = str.charAt(0);
   let restOfString = str.substring(1);
-  let result = [];
   let returnResult = getPermutations(restOfString);
 
   for (j = 0; j < returnResult.length; j++) {