decrypt(null) called when storepass/keypass is not configured

**Affected version:** HEAD

**Files:**
- `src/main/java/org/apache/maven/plugins/jarsigner/AbstractJarsignerMojo.java:540`
- `src/main/java/org/apache/maven/plugins/jarsigner/JarsignerSignMojo.java:313`

Both `processArchive()` and `createRequest()` call `decrypt(null)` when `storepass` or `keypass` are not configured:

```java
// AbstractJarsignerMojo.java:540
request.setStorepass(decrypt(storepass));

// JarsignerSignMojo.java:313
request.setKeypass(decrypt(keypass));
```

The `decrypt()` method passes this null to `securityDispatcher.decrypt(null)`. While the default `SecDispatcher` implementation may handle null gracefully, this is an unnecessary call that depends on implementation-specific behavior. The fix should skip the `decrypt()` call if the password is null.

Read processArchive() in src/main/java/org/apache/maven/plugins/jarsigner/AbstractJarsignerMojo.java and createRequest() in src/main/java/org/apache/maven/plugins/jarsigner/JarsignerSignMojo.java. Verify the password values before the decrypt() calls and confirm that null storepass or keypass values no longer reach securityDispatcher.decrypt(null).

Written by the indexing model from the issue text.