Handle empty feature element better for dev mode

[cherylking]

If a developer modifies their server.xml and accidentally leaves an empty <feature></feature> element in it, detect it and issue a warning on both dev mode start and restart, do not pass the empty feature to install-feature goal, and allow the process to continue. Currently, the following behavior happens:

1. If you start dev mode from the command line using mvn liberty:dev for a project that contains an empty feature element in server.xml, it fails.

[ERROR] Failed to execute goal io.openliberty.tools:liberty-maven-plugin:3.8:dev (default-cli) on project guide-getting-started: Error installing features for server defaultServer: CWWKF1299E: The following features could not be obtained: . Ensure that the features are valid for Open Liberty. -> [Help 1]

2. If you have dev mode running from command line, then edit the server.xml to add an empty feature element and save it...it gets an error but continues running dev mode.

[INFO] Installing features: [, jsonb-2.0, mphealth-4.0, mpconfig-3.0, mpmetrics-4.0, restfulws-3.0, jsonp-2.0, cdi-3.0]

[ERROR] Failed to install features from configuration file
org.apache.maven.plugin.MojoExecutionException: Error installing features for server defaultServer
    at io.openliberty.tools.maven.server.InstallFeatureMojo.doInstallFeatures (InstallFeatureMojo.java:77)
    at io.openliberty.tools.maven.server.InstallFeatureMojo.execute (InstallFeatureMojo.java:66)

The server shutdown works fine when I press q and hit enter key.

[cherylking]

Would need similar change in Liberty Gradle plugin.