Error when there is no file to process

[tonai]

When the task has no file to process, phpcs produces the following error :

Running "phpcs:app" (phpcs) task
ERROR: You must supply at least one file or directory to process.

Usage: phpcs [-nwlsaepvi] [-d key[=value]]
    [--report=] [--report-file=] [--report-=] ...
    [--report-width=] [--generator=] [--tab-width=]
    [--severity=] [--error-severity=] [--warning-severity=]
    [--runtime-set key value] [--config-set key value] [--config-delete key] [--config-show]
    [--standard=] [--sniffs=] [--encoding=]
    [--extensions=] [--ignore=]  ...
                      Set runtime value (see --config-set) 
        -n            Do not print warnings (shortcut for --warning-severity=0)
        -w            Print both warnings and errors (on by default)
        -l            Local directory only, no recursion
        -s            Show sniff codes in all reports
        -a            Run interactively
        -e            Explain a standard by showing the sniffs it includes
        -p            Show progress of the run
        -v[v][v]      Print verbose output
        -i            Show a list of installed coding standards
        -d            Set the [key] php.ini value to [value] or [true] if value is omitted
        --help        Print this help message
        --version     Print version information
                One or more files and/or directories to check
          A comma separated list of file extensions to check
                      (only valid if checking a directory)
            A comma separated list of patterns to ignore files and directories
            The encoding of the files being checked (default is iso-8859-1)
              A comma separated list of sniff codes to limit the check to
                      (all sniffs must be part of the specified standard)
            The minimum severity required to display an error or warning
            The name or path of the coding standard to use
            The number of spaces each tab represents
           The name of a doc generator to use
                      (forces doc generation instead of checking)
              Print either the "full", "xml", "checkstyle", "csv", "json"
                      "emacs", "source", "summary", "svnblame", "gitblame", "hgblame" or
                      "notifysend" report
                      (the "full" report is printed by default)
          Write the report to the specified file path
         How many columns wide screen reports should be printed
Warning: Task phpcs:app failed. Use --force to continue.

Aborted due to warnings.

which stops the grunt process.

Please add a test to skip the phpcs process when there is no file to process.
Thanks,