fix: set KEYPAD[0] to ""

Author: vil02

src/main/java/com/thealgorithms/strings/LetterCombinationsOfPhoneNumber.java

@@ -7,7 +7,7 @@
 public final class LetterCombinationsOfPhoneNumber {
 
     // Mapping of numbers to corresponding letters on a phone keypad
-    private static final String[] KEYPAD = new String[] {" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
+    private static final String[] KEYPAD = new String[] {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
 
     private LetterCombinationsOfPhoneNumber() {
     }