Validator for the MetaPhlAn2 element empty input slot

Scenario:

1. Open the WD.
2. Create a workflow: "Read FASTQ Files with PE Reads" > "Classify Sequences with MetaPhlAn2".
3. In the MetaPhlAn2 element set:
   • "Input data" to "PE reads".
   • "Input file format" to "FASTQ" (if requires, see also UGENE-6311).
4. In the "Input data" section on the Property Editor, change value of the second slot to empty.
5. Input a file to the first dataset of the the "Read FASTQ Files with PE Reads" element (not to the second one).
6. Run the workflow.
   Expected result: the workflow is NOT started, there is the following error in the "Error list" view at the bottom of the WD window:

      Classify Sequences with MetaPhlAn2: The mandatory "Input URL 2" slot is not connected.
      

   Current result: the workflow is started, there is a warning on the dashboard.

See similar behavior in other classifiers, e.g. Kraken.