RM: test-stage fails /with unstash exception/ if no installation/integration/acceptance test results are provided

[clemensutschig]

Is your feature request related to a problem? Please describe.
The RM teststage fails (non gracefully) if the test component does NOT return installation, integration and acceptance tests (as stashes)

java.lang.RuntimeException: Error: unable to unstash JUnit XML reports, type 'installation' for repo 'tosca' from stash 'installation-test-reports-junit-xml-tosca-127'.
at org.ods.orchestration.Stage.getTestResults(Stage.groovy:115)
at org.ods.orchestration.TestStage.run(TestStage.groovy:40)
at com.cloudbees.groovy.cps.CpsDefaultGroovyMethods.callClosureForMapEntry(CpsDefaultGroovyMethods:5228)
at com.cloudbees.groovy.cps.CpsDefaultGroovyMethods.each(CpsDefaultGroovyMethods:2107)
at org.ods.orchestration.TestStage.run(TestStage.groovy:39)
at org.ods.orchestration.util.MROPipelineUtil.prepareExecutePhaseForRepoNamedJob(MROPipelineUtil.groovy:407)
at org.ods.orchestration.util.PipelineUtil.executeBlockAndFailBuild(PipelineUtil.groovy:154)
at org.ods.orchestration.util.MROPipelineUtil.prepareExecutePhaseForRepoNamedJob(MROPipelineUtil.groovy:333)
at ___cps.transform___(Native Method)

Describe the solution you'd like
In case no stash results are provided, a warning should be displayed - and the error should come from JIRA (unexecuted tests) - but not from within the teststage