rebaseline.py: fix --add-new when there are no expectations at all

tools/tests/rebaseline/output/add-new/output-expected/gm-expectations/Test-Android-GalaxyNexus-SGX540-Arm7-Debug/expected-results.json

 {
   "expected-results": {
     "3x3bitmaprect_565.png": {
-      "allowed-digests": null, 
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          16998423976396106083
+        ]
+      ], 
       "ignore-failure": false
     }, 
     "3x3bitmaprect_8888.png": {
-      "allowed-digests": null, 
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          2054956815327187963
+        ]
+      ], 
       "ignore-failure": false
     }, 
     "aaclip_gpu.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           11899819492385205974
         ]
       ], 
       "ignore-failure": false, 
@@ skipping 30 matching lines @@
     "shadertext3_8888.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           3713708307125704716
         ]
       ], 
       "ignore-failure": false
     }, 
     "xfermodeimagefilter_pdf.png": {
-      "allowed-digests": null, 
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          16502178848783208088
+        ]
+      ], 
       "ignore-failure": false
     }, 
     "xfermodes_pdf.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           9151974350149210736
         ]
       ], 
       "ignore-failure": false
     }
   }
 }