Incorrect handling of NA character values in DataFrame / DynamicVector

[marymoni]

It seems that there is a bug with string values handling in DynamicVector class - when trying to get NA value, it is converted into 'NA' string and not in null value (like CharacterVector does).

Here is code to reproduce when working with DynamicVector's via DataFrame class:

private static void TestNa(REngine engine)
{

    engine.Evaluate("options(stringsAsFactors = FALSE)");

    var df = engine.Evaluate("data.frame(col_str = c('AAA', NA_character_))").AsDataFrame();
    
    var col_str = df["col_str"];
    
    var val1 = col_str[1]; // 'NA' - not expected
    
    var val2 = col_str.AsCharacter()[1]; // null - expected

}

It seems that method DynamicVector.ReadString() misses check for NaStringPointer which exists in CharacterVector.GetValue() method.