fix: add jspecify annotations

Author: jeremylong

src/main/java/com/github/packageurl/internal/StringUtil.java

@@ -25,6 +25,7 @@
 
 import com.github.packageurl.ValidationException;
 import java.nio.charset.StandardCharsets;
+import org.jspecify.annotations.NonNull;
 
 /**
  * String utility for validation and encoding.
@@ -65,7 +66,7 @@ private StringUtil() {
      *
      * @since 2.0.0
      */
-    public static String toLowerCase(String s) {
+    public static @NonNull String toLowerCase(@NonNull String s) {
         int pos = indexOfFirstUpperCaseChar(s);
 
         if (pos == -1) {
@@ -89,7 +90,7 @@ public static String toLowerCase(String s) {
      *
      * @since 2.0.0
      */
-    public static String percentDecode(final String source) {
+    public static @NonNull String percentDecode(@NonNull final String source) {
         if (source.indexOf(PERCENT_CHAR) == -1) {
             return source;
         }
@@ -120,7 +121,7 @@ public static String percentDecode(final String source) {
      *
      * @since 2.0.0
      */
-    public static String percentEncode(final String source) {
+    public static @NonNull String percentEncode(@NonNull final String source) {
         if (!shouldEncode(source)) {
             return source;
         }
@@ -142,10 +143,6 @@ public static String percentEncode(final String source) {
         return new String(dest, 0, writePos, StandardCharsets.UTF_8);
     }
 
-    private static byte toHexDigit(int b) {
-        return (byte) Character.toUpperCase(Character.forDigit(b & 0xF, 16));
-    }
-
     /**
      * Determines if the character is a digit.
      *
@@ -182,6 +179,10 @@ public static boolean isValidCharForKey(int c) {
         return (isAlphaNumeric(c) || c == '.' || c == '_' || c == '-');
     }
 
+    private static byte toHexDigit(int b) {
+        return (byte) Character.toUpperCase(Character.forDigit(b & 0xF, 16));
+    }
+
     /**
      * Returns {@code true} if the character is in the unreserved RFC 3986 set.
      * <p>

src/main/java/com/github/packageurl/internal/package-info.java

@@ -23,4 +23,7 @@
 /**
  * This package contains utility classes used by the PackageURL library.
  */
+@NullMarked
 package com.github.packageurl.internal;
+
+import org.jspecify.annotations.NullMarked;