What this PR does

When developers run ./gradlew publishToMavenLocal on their local machines without configuring GPG signing credentials (signing.keyId, signing.secretKeyRingFile), the build fails with:

Invalid publication 'maven': artifact file does not exist: '.../release-javadoc.jar.asc'

This occurred because signAllPublications() in PublishingConventionPlugin.kt was invoked unconditionally.

This PR wraps signAllPublications() so that it only signs when signing.keyId or signing.secretKeyRingFile is present and non-empty:

val isSigningConfigured = !providers.gradleProperty("signing.keyId").getOrElse("").isEmpty() ||
    !providers.gradleProperty("signing.secretKeyRingFile").getOrElse("").isEmpty()
if (isSigningConfigured) {
    signAllPublications()
}

Verification

• Executed ./gradlew publishToMavenLocal across all modules: BUILD SUCCESSFUL without .asc signature errors.
• Verified that all 7 library modules are correctly installed in ~/.m2/repository/com/google/maps/android/.