don't fail the build if the static folder is missing or empty

Fixes #13.
The build will log a warning for missing folders, but still continue

    my_host:build my_user$ ant build_modular_input
    Buildfile: /my_repo/SplunkModularInputsPythonFramework/build/build.xml
    
    init:
        [mkdir] Created dir: /my_repo/SplunkModularInputsPythonFramework/temp_build
    
    build_modular_input:
         [copy] Copying 1 file to /my_repo/SplunkModularInputsPythonFramework/temp_build/metadata
         [copy] Copying 5 files to /my_repo/SplunkModularInputsPythonFramework/temp_build/bin
         [copy] Copying 4 files to /my_repo/SplunkModularInputsPythonFramework/temp_build/appserver
         [copy] Copying 4 files to /my_repo/SplunkModularInputsPythonFramework/temp_build/default
         [copy] Copying 1 file to /my_repo/SplunkModularInputsPythonFramework/temp_build/README
         [copy] Warning: /my_repo/SplunkModularInputsPythonFramework/implementations/snmp/static does not exist.
          [tar] Building tar: /my_repo/SplunkModularInputsPythonFramework/releases/snmp_ta-1.2.7.spl
       [delete] Deleting directory /my_repo/SplunkModularInputsPythonFramework/temp_build
    
    BUILD SUCCESSFUL
    Total time: 0 seconds