Fix crash on episode list

[cewert]

Prevent crash by validating all data sent to CreateSeasonDetailsGroupByID(). Comes from roku.com crash log:

group            <uninitialized> 
seasonmetadata   <uninitialized> 
m                roAssociativeArray refcnt=3 count:6 
global           Interface:ifGloba$1 seasonid         Invali$1 seriesid         roString (2.1 was String) refcnt=1 val:"9f3800b10bae7a29a8bfbfc6d5b4f32e" 
Local Variables: 
   file/line: pkg:/source/Main.brs(786) 
#0  Function main(args As Dynamic) As Voi$1 file/line: pkg:/source/ShowScenes.brs(793) 
#1  Function createseasondetailsgroupbyid(seriesid As String, seasonid As String) As Dynami$1 Backtrace: 
Type Mismatch. Unable to cast "Invalid" to "String". (runtime error &h18) in pkg:/source/ShowScenes.brs(791)

which points to this line after running build-prod on 2.1.2:

function CreateSeasonDetailsGroupByID(seriesID as string, seasonID as string) as dynamic

Issues

Ref #1164