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

tools/tests/rebaseline/input/json1/Test-Mac10.6-MacMini4.1-GeForce320M-x86-Release/actual-results.json

 {
    "actual-results" : {
-      "failed" : {
-         "aaclip_565.png" : [ "bitmap-64bitMD5", 6190901827590820995 ],
-         "aaclip_8888.png" : [ "bitmap-64bitMD5", 14456211900777561488 ]
-      },
+      "failed" : null,
       "failure-ignored" : null,
       "no-comparison" : {
+         "aaclip_565.png" : [ "bitmap-64bitMD5", 6190901827590820995 ],
+         "aaclip_8888.png" : [ "bitmap-64bitMD5", 14456211900777561488 ],
+         "aaclip_gpu.png" : [ "bitmap-64bitMD5", 11899819492385205974 ],
          "aaclip_pdf.png" : [ "bitmap-64bitMD5", 13515207506164874564 ]
       },
-      "succeeded" : {
-         "aaclip_gpu.png" : [ "bitmap-64bitMD5", 11899819492385205974 ]
-      }
+      "succeeded" : null
    },
    "expected-results" : {
       "aaclip_565.png" : {
-         "allowed-digests" : [
-            [ "bitmap-64bitMD5", 12345 ]
-         ],
+         "allowed-digests" : null,
          "ignore-failure" : false
       },
       "aaclip_8888.png" : {
-         "allowed-digests" : [
-            [ "bitmap-64bitMD5", 67890 ]
-         ],
+         "allowed-digests" : null,
          "ignore-failure" : false
       },
       "aaclip_gpu.png" : {
-         "allowed-digests" : [
-            [ "bitmap-64bitMD5", 11899819492385205974 ]
-         ],
+         "allowed-digests" : null,
          "ignore-failure" : false
       },
       "aaclip_pdf.png" : {
          "allowed-digests" : null,
          "ignore-failure" : false
       }
    }
 }