feat: Add JSpecify nullability annotations (package-url#175)

* Add nullability annotations

This change adds nullability annotations ([JSpecify](https://jspecify.dev)) to `packageurl-java` and modifies somes behavior. Specifically:

- It marks with `@Nullable` all public methods that can return `null`.
- It marks with `@Nullable` all method parameters that can have a `null` value **without** throwing any exception.
- It modifies the behavior of the remaining parameters: passing `null` to those parameters will result in an NPE, instead of `MalformedPackageURLException`. This is at least a behavior change (minor version bump), but it _might_ be a breaking change.

* Fix qualifier semantics

Author: ppkarwasz

pom.xml

@@ -111,6 +111,11 @@
             <optional>true</optional>
             <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>org.jspecify</groupId>
+            <artifactId>jspecify</artifactId>
+            <version>1.0.0</version>
+        </dependency>
         <dependency>
             <groupId>junit</groupId>
             <artifactId>junit</artifactId>

src/main/java/com/github/packageurl/MalformedPackageURLException.java

@@ -21,6 +21,8 @@
  */
 package com.github.packageurl;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Exception class intended to be used for PackageURL parsing exceptions.
  *
@@ -42,7 +44,7 @@ public MalformedPackageURLException() {
      *
      * @param msg the detail message.
      */
-    public MalformedPackageURLException(String msg) {
+    public MalformedPackageURLException(@Nullable String msg) {
         super(msg);
     }