Improvement to increase stability of repository search

[delcroip]

this call to repository search gives error when running PD/$apply for smart immunization measles :

Parameter specified as non-null is null: method com.google.android.fhir.workflow.testing.IGInputStreamStructureRepository.search, parameter headers

this happend because the Repository class IGInputStreamStructureRepository did not set headers as nullable

Empty headers are specified with Collections.emptyMap() in other part of the code such as here