cmd/go/internal/modcmd: assign module's path and version to fileJSON when...

cmd/go/internal/modcmd: assign module's path and version to fileJSON when modFile's module statement exists

Fixes panic on nil pointer dereference error when assigning module's path and version pair to fileJSON.

Fixes #31623

Change-Id: I3f61122ba0676a1270d3ad98900af8c8e9c90935
Reviewed-on: https://go-review.googlesource.com/c/go/+/173397
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modcmd/edit.go

@@ -385,7 +385,9 @@ type replaceJSON struct {
 // editPrintJSON prints the -json output.
 func editPrintJSON(modFile *modfile.File) {
 	var f fileJSON
-	f.Module = modFile.Module.Mod
+	if modFile.Module != nil {
+		f.Module = modFile.Module.Mod
+	}
 	if modFile.Go != nil {
 		f.Go = modFile.Go.Version
 	}

src/cmd/go/testdata/script/mod_edit.txt

@@ -25,6 +25,10 @@ cmpenv go.mod $WORK/go.mod.edit2
 go mod edit -json
 cmpenv stdout $WORK/go.mod.json
 
+# go mod edit -json (empty mod file)
+go mod edit -json $WORK/go.mod.empty
+cmp stdout $WORK/go.mod.empty.json
+
 # go mod edit -replace
 go mod edit -replace=x.1@v1.3.0=y.1/v2@v2.3.5 -replace=x.1@v1.4.0=y.1/v2@v2.3.5
 cmpenv go.mod $WORK/go.mod.edit3
@@ -168,4 +172,14 @@ require x.3   v1.99.0
 -- $WORK/m/go.mod.edit --
 module x.x/y/z
 
-go $goversion
\ No newline at end of file
+go $goversion
+-- $WORK/go.mod.empty --
+-- $WORK/go.mod.empty.json --
+{
+	"Module": {
+		"Path": ""
+	},
+	"Require": null,
+	"Exclude": null,
+	"Replace": null
+}