java: Fix samples using SCIP

ortools/linear_solver/samples/AssignmentMip.java

@@ -42,6 +42,10 @@ public class AssignmentMip {
     // [START solver]
     // Create the linear solver with the SCIP backend.
     MPSolver solver = MPSolver.createSolver("SCIP");
+    if (solver == null) {
+      System.out.println("Could not create solver SCIP");
+      return;
+    }
     // [END solver]
 
     // Variables