Failed to convert from cml file in case of only rgroup is present without rgroup reference

**Steps to reproduce:**

Send POST request to https://lifescience.opensource.epam.com/v2/indigo/convert
with following payload
`{"struct":"\n\n \n \n \n \n \n \n \n \n \n \n \n \n\n","output_format":"chemical/x-mdl-molfile","options":{"smart-layout":true,"ignore-stereochemistry-errors":true,"mass-skip-error-on-pseudoatoms":false,"gross-formula-add-rsites":true}}`
**Actual result:**
502 Bad Gateway Response status is returned

**Expected result:**
Correct rgroup molecule description, i.e.
`$MDL REV 1
$MOL
$HDR

$END HDR
$CTAB
0 0 0 0 0 999 V2000
M END
$END CTAB
$RGP
5
$CTAB
2 1 0 0 0 999 V2000
7.7000 -2.0750 0.0000 C 0 0 0 0 0 0 0 0 0 0 0 0
6.9929 -1.3679 0.0000 C 0 0 0 0 0 0 0 0 0 0 0 0
1 2 1 0 0 0
M END
$END CTAB
$END RGP
$END MOL`