Yield null from cy.writeFile (#2731)

- Fixes #2466 
- [Docs PR](cypress-io/cypress-documentation#1117)

Should be part of 4.0.0

Author: lilaconlee

packages/driver/src/cy/commands/files.coffee

@@ -94,16 +94,13 @@ module.exports = (Commands, Cypress, cy, state, config) ->
         })
 
       if _.isObject(contents)
-        objContents = contents
         contents = JSON.stringify(contents, null, 2)
 
       Cypress.backend("write:file", fileName, contents, _.pick(options, ["encoding", "flag"]))
       .then ({ contents, filePath }) ->
         consoleProps["File Path"] = filePath
         consoleProps["Contents"] = contents
-        if objContents?
-          return objContents
-        return contents
+        return null
       .catch Promise.TimeoutError, (err) ->
         $utils.throwErrByPath "files.timed_out", {
           onFail: options._log

packages/driver/test/cypress/integration/commands/files_spec.coffee

@@ -286,17 +286,11 @@ describe "src/cy/commands/files", ->
           }
         )
 
-    it "sets the contents as the subject", ->
+    it "yields null", ->
       Cypress.backend.resolves(okResponse)
 
       cy.writeFile("foo.txt", "contents").then (subject) ->
-        expect(subject).to.equal("contents")
-       
-    it "sets a JSON as the subject", ->
-      Cypress.backend.resolves(okResponse)
-
-      cy.writeFile("foo.json", { name: "Test" }).then (subject) ->
-        expect(subject.name).to.equal("Test")
+        expect(subject).to.not.exist
 
     it "can write a string", ->
       Cypress.backend.resolves(okResponse)