REFAC: return an empty projection in GetGCPProjection if the image has no gcp...

REFAC: return an empty projection in GetGCPProjection if the image has no gcp instead of asserting that the image has gcps (or segfault in release mode). This means that the user of ImageCommon does not have to check that the image has gcp before calling this method

Modules/Core/ImageBase/src/otbImageCommons.cxx

@@ -54,8 +54,11 @@ void ImageCommons::SetProjectionRef(const std::string& proj)
 
 std::string ImageCommons::GetGCPProjection(void) const
 {
-  assert(m_Imd.Has(MDGeom::GCP));
-  return m_Imd.GetGCPParam().GCPProjection;
+  if (m_Imd.Has(MDGeom::GCP))
+  {
+    return m_Imd.GetGCPParam().GCPProjection;
+  }
+  return "";
 }