Do not set test.fmf.name attribute if null

This is causing an issue for some of the users, if they do not set the
plan regex:

```
Run sclorg/testing-farm-as-github-action@v2
19 Error: [
20  {
21    "code": "too_small",
22    "minimum": 1,
23    "type": "string",
24    "inclusive": true,
25    "exact": false,
26    "message": "String must contain at least 1 character(s)",
27    "path": [
28      "test",
29      "fmf",
30      "name"
31    ]
32  }
33]
```

From:

https://github.com/osbuild/bootc-image-builder/actions/runs/7874729084/job/21520477899?pr=194

Signed-off-by: Miroslav Vadkerti <mvadkert@redhat.com>

src/action.ts

@@ -90,7 +90,8 @@ async function action(pr: PullRequest): Promise<void> {
       fmf: {
         url: getInput('git_url', { required: true }),
         ref: getInput('git_ref'),
-        name: getInput('tmt_plan_regex'),
+        // Conditionally include the name attribute only if tmt_plan_regex is not null
+        ...(getInput('tmt_plan_regex') ? { name: getInput('tmt_plan_regex') } : {}),
       },
     },
     environments: [