Fix nil ptr bug in checkGenerated (#159)

Currently, while checking if a file contains generated code, if the file
doesn't contain any package comment,
 we run into a nil pointer dereference error.
```
package example
... // rest of the logic
```
**Why**
This happens due to missing nil check in the `checkGenerated` function
before iterating `ast.File.Doc.List` ([pkg
ref](https://pkg.go.dev/go/ast#File))
```
for _, comm := range f.Doc.List { // nil pointer error when f.Doc is nil
```
This commit adds a nil pointer check in `checkGenerated` to continue
with the rest of the logic only if there's a package comment.

main.go

@@ -355,6 +355,9 @@ func checkGeneratedCode(f *ast.File) bool {
 	if ast.IsGenerated(f) {
 		return true
 	}
+	if f.Doc == nil {
+		return false
+	}
 	for _, comm := range f.Doc.List {
 		if strings.Contains(comm.Text, "@generated") {
 			return true

main_test.go

@@ -252,6 +252,10 @@ func TestCheckGeneratedCode(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "non-generated code with no package comment",
+			file: &ast.File{},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {