fix: return null instead of crashing when no solution is found

spec/cube.spec.coffee

@@ -58,3 +58,9 @@ describe 'Cube', ->
     Cube.initSolver()
     cube = new Cube
     expect(cube.solve()).toBe "R L U2 R L F2 R2 U2 R2 F2 R2 U2 F2 L2"
+
+  # ignore because Travis is slow
+  xit 'should return null if no solution is found (maxDepth too low)', ->
+    Cube.initSolver()
+    cube = Cube.random()
+    expect(cube.solve(1)).toBe null

src/solve.coffee

@@ -662,6 +662,7 @@ Cube::solveUpright = (maxDepth=22) ->
   phase1search(state)
   freeStates.push(state)
 
+  return null if not solution?
   # Trim the trailing space and return
   solution.trim()
 
@@ -687,6 +688,7 @@ Cube::solve = (maxDepth=22) ->
   clone.move upright
   rotation = new Cube().move(upright).center
   uprightSolution = clone.solveUpright maxDepth
+  return null if not uprightSolution?
   solution = []
   for move in uprightSolution.split ' '
     solution.push faceNames[rotation[faceNums[move[0]]]]