#5254 Support estimating RKB-diff from TVDMSL and MDRKB when first TVDMSL == 0

* Add warning when the RKB-diff cannot be estimated

ApplicationCode/ReservoirDataModel/RigWellPath.cpp

@@ -77,6 +77,10 @@ double RigWellPath::rkbDiff() const
 
             return diff;
         }
+        else if ( cvf::Math::abs( m_wellPathPoints[0].z() ) < epsilon )
+        {
+            return m_measuredDepths[0]; // Assume a vertical drop before the first md point.
+        }
     }
     return HUGE_VAL;
 }