fix: address edge-cases related to --all when instrumentation is disabled (#482)

Author: wbyoung

lib/instrumenters/noop.js

@@ -3,12 +3,12 @@ var readInitialCoverage = require('istanbul-lib-instrument').readInitialCoverage
 
 function NOOP () {
   return {
-    instrumentSync: function (code) {
+    instrumentSync: function (code, filename) {
       var extracted = readInitialCoverage(code)
       if (extracted) {
         this.fileCoverage = new FileCoverage(extracted.coverageData)
       } else {
-        this.fileCoverage = null
+        this.fileCoverage = new FileCoverage(filename)
       }
       return code
     },