ShaderNetwork : Fix bug causing extra shader copying when substituting

Author: danieldresser-ie

Changes

@@ -1,7 +1,10 @@
 10.6.x.x (relative to 10.6.1.0)
 ========
 
+Improvements
+------------
 
+- ShaderNetwork : Optimised applySubstitutions() for cases where not all shaders need them.
 
 10.6.1.0 (relative to 10.6.0.2)
 ========

src/IECoreScene/ShaderNetwork.cpp

@@ -745,7 +745,10 @@ class ShaderNetwork::Implementation
 						}
 					}
 
-					m_parmsNeedingSubstitution[ node.handle ] = parmsNeedingSub;
+					if( parmsNeedingSub.size() )
+					{
+						m_parmsNeedingSubstitution[ node.handle ] = parmsNeedingSub;
+					}
 				}
 
 				m_hash.append( m_output.shader );