MongoDB string functions - different handling of NULL values

Teiid doesn't handle NULL values right in given functions when working with mongodb.

• SUBSTRING(<column resolving to NULL>,2)
  • query:

    SELECT INTKEY, STRINGNUM, SUBSTRING(STRINGNUM, 2) FROM BQT1.SmallA ORDER BY INTKEY
    

  • source query:
    Unable to find source-code formatter for language: plain. Available languages are: actionscript, ada, applescript, bash, c, c#, c++, cpp, css, erlang, go, groovy, haskell, html, java, javascript, js, json, lua, none, nyan, objc, perl, php, python, r, rainbow, ruby, scala, sh, sql, swift, visualbasic, xml, yaml

    {"$project": {{ "c_0" : "$INTKEY" , "c_1" : "$STRINGNUM" , "c_2" : { "$substr" : [ "$STRINGNUM" , { "$subtract" : [ 2 , 1]} , 4000]}}}}
    

  • problem:
    For row with NULL value in given column returns sth like:
    Unable to find source-code formatter for language: plain. Available languages are: actionscript, ada, applescript, bash, c, c#, c++, cpp, css, erlang, go, groovy, haskell, html, java, javascript, js, json, lua, none, nyan, objc, perl, php, python, r, rainbow, ruby, scala, sh, sql, swift, visualbasic, xml, yaml

    { "_id" : ObjectId("534bf17516997a2a41000002"), "c_0" : 1, "c_1" : null, "c_2" : "" }
    

    which results in "" as result for SUBSTRING(NULL, 2) which should return NULL instead

• UCASE, LCASE (UPPER, LOWER)
  • query:

    SELECT intkey, stringnum, LOWER(stringnum) AS LOWER FROM BQT1.SmallA ORDER BY intkey
    

  • source query:
    Unable to find source-code formatter for language: plain. Available languages are: actionscript, ada, applescript, bash, c, c#, c++, cpp, css, erlang, go, groovy, haskell, html, java, javascript, js, json, lua, none, nyan, objc, perl, php, python, r, rainbow, ruby, scala, sh, sql, swift, visualbasic, xml, yaml

    {"$project": {{ "c_0" : "$INTKEY" , "c_1" : "$STRINGNUM" , "c_2" : { "$toLower" : [ "$STRINGNUM"]}}}}
    

  • problem:
    For row with NULL value in given column returns sth like:
    Unable to find source-code formatter for language: plain. Available languages are: actionscript, ada, applescript, bash, c, c#, c++, cpp, css, erlang, go, groovy, haskell, html, java, javascript, js, json, lua, none, nyan, objc, perl, php, python, r, rainbow, ruby, scala, sh, sql, swift, visualbasic, xml, yaml

    { "_id" : ObjectId("534bf17516997a2a41000002"), "c_0" : 1, "c_1" : null, "c_2" : "" }
    

    which results in "" as result for LCASE(NULL) which should return NULL instead