PERF: Prefer default initialization for std::string

Return {} may be the fastest, because it just calls the
default-constructor of std::string.  (return { "" } and return "" may be
slightly slower because they convert a literal string to an std::string.

This probably does not have a substantive impact on the current
uses, but implementing to keep the style consistent.

Modules/Core/Common/src/itkBuildInformation.cxx.in

@@ -87,7 +87,7 @@ BuildInformation::GetValue(const MapKeyType & key)
   {
     return it->second.m_Value;
   }
-  return {""};
+  return {};;
 }
 
 const BuildInformation::MapValueDescriptionType
@@ -100,7 +100,7 @@ BuildInformation::GetDescription(const MapKeyType & key)
   {
     return it->second.m_Description;
   }
-  return {""};
+  return {};;
 }
 
 const std::vector<  BuildInformation::MapKeyType >

Modules/Core/SpatialObjects/include/itkDTITubeSpatialObjectPoint.hxx

@@ -64,7 +64,7 @@ DTITubeSpatialObjectPoint<TPointDimension>::TranslateEnumToChar(DTITubeSpatialOb
       // Just fall through.
       break;
   }
-  return { "" };
+  return {};
 }
 
 template <unsigned int TPointDimension>

Modules/IO/SpatialObjects/src/itkPolygonGroupSpatialObjectXMLFile.cxx

@@ -165,7 +165,7 @@ class NumericTraits<std::string>
   static std::string
   ZeroValue()
   {
-    return { "" };
+    return {};
   }
 };
 

Modules/Numerics/Optimizers/src/itkLBFGSOptimizer.cxx

@@ -315,6 +315,6 @@ LBFGSOptimizer::GetStopConditionDescription() const
     return m_StopConditionDescription.str();
   }
 
-  return { "" };
+  return {};
 }
 } // end namespace itk

Modules/Numerics/Optimizersv4/src/itkLBFGSOptimizerv4Base.cxx

@@ -213,7 +213,7 @@ LBFGSOptimizerBasev4<TInternalVnlOptimizerType>::GetStopConditionDescription() c
     return m_StopConditionDescription.str();
   }
 
-  return { "" };
+  return {};
 }
 
 template class ITKOptimizersv4_EXPORT LBFGSOptimizerBasev4<vnl_lbfgs>;