Missing null check on artifact parameter in evaluateFileNameMapping

## Description

`MappingUtils.evaluateFileNameMapping(String expression, Artifact artifact)` does not validate that the `artifact` parameter is non-null. If a caller passes `null`, a `NullPointerException` is thrown at lines 91-95 when calling `artifact.getArtifactHandler()` and `artifact.getClassifier()`.

## Expected behavior

A clear `NullPointerException` should be thrown with a descriptive message, rather than an opaque NPE deep in the call stack.

## Relevant code

`src/main/java/org/apache/maven/shared/mapping/MappingUtils.java:88-98`

Open src/main/java/org/apache/maven/shared/mapping/MappingUtils.java and start at evaluateFileNameMapping(String expression, Artifact artifact), around lines 88-98. Confirm the null artifact path and make sure a null argument produces a clear, descriptive NullPointerException rather than failing at artifact.getArtifactHandler() or artifact.getClassifier().

Written by the indexing model from the issue text.