[Infra] Only set commitsToIgnoreFile convention if file exists (#8479)

Updated `PublishingPlugin` to verify the existence of
`ignoreCommits.txt` before setting it as the `commitsToIgnoreFile`
convention.

Author: rlazo

plugins/src/main/java/com/google/firebase/gradle/plugins/PublishingPlugin.kt

@@ -665,8 +665,9 @@ abstract class PublishingPlugin : Plugin<Project> {
       currentRelease.convention(project.provideProperty("currentRelease"))
       pastRelease.convention(project.provideProperty("pastRelease"))
       printReleaseConfig.convention(project.provideProperty("printOutput"))
-      commitsToIgnoreFile.convention(project.layout.projectDirectory.file("ignoreCommits.txt"))
-
+      project.layout.projectDirectory.file("ignoreCommits.txt").let {
+        if (it.asFile.exists()) commitsToIgnoreFile.convention(it)
+      }
       releaseConfigFile.convention(project.layout.projectDirectory.file(RELEASE_CONFIG_FILE))
       releaseReportJsonFile.convention(
         project.layout.projectDirectory.file(RELEASE_REPORT_JSON_FILE)