Vector Assembler with no input columns leads to opaque error

When `VectorAssembler ` is given an empty array as its inputColumns it throws an opaque error. In versions less than 2.3 `VectorAssembler` it simply appends a column containing empty vectors. 

val inputCols = Array()
val outputCols = Array("A")
val vectorAssembler = new VectorAssembler()
.setInputCols(inputCols)
.setOutputCol(outputCols)

vectorAssmbler.fit(data).transform(df)

In versions 2.3 > this throws the exception below

org.apache.spark.sql.AnalysisException: cannot resolve 'named_struct()' due to data type mismatch: input to function named_struct requires at least one argument;;

Whereas in versions less than 2.3 it just adds a column containing an empty vector.

I'm not certain if this is an intentional choice or an actual bug. If this is a bug, the `VectorAssembler` should be modified to append an empty vector column if it detects no inputCols.

If it is a design decision it would be nice to throw a human readable exception explicitly stating inputColumns must not be empty. The current error is somewhat opaque.