CreatePom throws NullPointerException when pomTarget has no parent directory

## Summary
`CreatePom` throws a `NullPointerException` when `pomTarget` points at a file with no parent directory (e.g. `pomTarget="pom.xml"`).

## Affected code
`src/main/java/org/apache/maven/resolver/internal/ant/tasks/CreatePom.java` lines 332-335 (master @ `df2908e`)

```java
@Override
public void execute() {
if (!pomFile.getParentFile().exists()) {
pomFile.getParentFile().mkdirs();
}
```

## Problem
When `pomTarget` is a bare relative filename such as `pom.xml`, `new File("pom.xml").getParentFile()` returns `null`, so `pomFile.getParentFile().exists()` throws an NPE. Only `pomTarget` values containing a directory component (e.g. `out/pom.xml`) work.

## Expected behavior
A bare filename should be written to the current working directory (skip the parent-dir creation, or treat a null parent as the base dir).

## Reproduction
```xml

```

Start in src/main/java/org/apache/maven/resolver/internal/ant/tasks/CreatePom.java at execute(), especially lines 332-335, and inspect how pomFile is initialized from pomTarget. Reproduce with pomTarget="pom.xml" and verify that a bare filename is written in the current working directory without an exception, while targets with directory components still work.

Written by the indexing model from the issue text.