Fix: do not add section_system if there are no topology or trajectory files

parser/parser-amber/AMBERParser.py

@@ -612,6 +612,8 @@ class AMBERParser(AmberC.AMBERParserBase):
         self.metaStorage.updateBackend(backend.superBackend, 
                 startsection=['section_single_configuration_calculation'],
                 autoopenclose=False)
+        if(self.topology is not None or 
+           self.atompositions is not None):
             self.onOpen_section_system(backend, None, None)
             self.onClose_section_system(backend, None, None)
         backend.superBackend.closeSection("section_single_configuration_calculation", self.secSingleGIndex)