Category validation fails when there is no categorical variable

[ymarcon]

• choose a table without categorical variables
• create a new analysis, with "Categories validator" type
• run

The error stack trace is:

org.obiba.opal.spi.r.REvaluationRuntimeException: Error while evaluating 'rmarkdown::render('report.Rmd')' -> Error in `$<-.data.frame`(`*tmp*`, "description", value = "") : 
  replacement has 1 row, data has 0

	at org.obiba.opal.spi.r.AbstractROperation.eval(AbstractROperation.java:123) ~[opal-spi-r-2.14-SNAPSHOT.jar:na]
	at org.obiba.opal.spi.r.AbstractROperationWithResult.eval(AbstractROperationWithResult.java:68) ~[opal-spi-r-2.14-SNAPSHOT.jar:na]
	at org.obiba.opal.spi.r.AbstractROperation.eval(AbstractROperation.java:96) ~[opal-spi-r-2.14-SNAPSHOT.jar:na]
	at org.obiba.opal.spi.r.AbstractROperationWithResult.eval(AbstractROperationWithResult.java:63) ~[opal-spi-r-2.14-SNAPSHOT.jar:na]
	at org.obiba.analysis.opal.validate.ValidateAnalysisROperation.doWithConnection(ValidateAnalysisROperation.java:33) ~[opal-analysis-validate-1.1-SNAPSHOT.jar:na]

Expected result is success.