Handle NullPointerException

[sridamul]

Closes #101

Testing done

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[gounthar]

We are currently losing important information about missing files in our error messages. While the system informs users that no plugins were found, (Error reading plugins from file: plugins.txt) it fails to explicitly state that this is due to a missing file. Should we go to this level of details?

java -jar plugin-modernizer-cli/target/jenkins-plugin-modernizer-999999-SNAPSHOT.jar --dry-run --plugin-file plugins.txt  --recipes AddPluginsBom,AddCodeOwner 
Picked up JAVA_TOOL_OPTIONS: -XX:+UseContainerSupport -XX:ActiveProcessorCount=1
Error reading plugins from file: plugins.txt 
Starting Plugin Modernizer 
Picked up JAVA_TOOL_OPTIONS: -XX:+UseContainerSupport -XX:ActiveProcessorCount=1
Plugins: [] 
Recipes: [AddPluginsBom, AddCodeOwner] 
GitHub owner: gounthar 

[gounthar]

Thanks, this works for me.