do not call schema converter to generate projected schema when projection is not set

do not call schema converter to generate projected schema when projection is not set, so:
if the user do not set projection filter, ThriftConverter and ScroogeConverter will not be called, and the MessageType  stored in the parquet file will be used as the schema. 

Part of the reason of this PR is to limit the usage of ScroogeSchemaConverter, since some problems are caused by the inaccuracy from ScroogeStructConverter due to its usage of reflection. One example is if you have an enum named as FOO_BAR_1, scrooge will rename it to FooBar1.