Update optimade.rst

Co-authored-by: Antanas Vaitkus <antanas.vaitkus90@gmail.com>

optimade.rst

@@ -3536,7 +3536,7 @@ The :entry:`calculations` entries have the properties described above in section
 Related :entry:`files` entries specified as relationships (as described in section `Relationships`_) list files used or produced by a calculation.
 They MAY be defined as either input or output files by the :field:`role` field inside the :field:`meta` dictionary of the JSON:API resource identifier object.
 The allowed values are :val:`input` for input files and :val:`output` for output files.
-Absence of :field:`role` or :val:`null` means that the role is undefined and SHOULD NOT be assumed.
+If the :field:`role` field is not provided or the value is set to :val:`null`, a default role SHOULD NOT be assumed.
 It is RECOMMENDED to order the output files in the list of calculation's relationships in a way to prioritize the files most likely to contain details which could be used to automatically analyze the performed calculation.
 This way output parsing software would need to read in a small number of calculation output files to possibly identify the code, type and possibly parameters of calculation performed.