fix: occasional panic when watched YAML files change (#1246)

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## This PR
At the moment, the yamlToJSON will convert `[]byte("")` to `"null"`.
While "null" is a normal string, it will bypass all validator and lead
to panic.
### Related Issues
<!-- add here the GitHub issue that this PR resolves if applicable -->

Fixes [#1242](https://github.com/open-feature/flagd/issues/1242)

### Notes
I found that sometime, the os.Readfile will return empty byte and no
error, this will cause the yamlToJSON convert the rawData ("") to
jsonData ("null").
Moreover, when I check the log, the watcher is reading file content a
lot at a times for checking file changes, so it could be the reason why
os.Readfile return empty byte?

![image](https://github.com/open-feature/flagd/assets/20945393/75b68be0-eba4-4f96-b5ee-5f4f383bae5c)
### Follow-up Tasks
I think we have to check the behavior of os.Readfile?

### How to test
<!-- if applicable, add testing instructions under this section -->

Signed-off-by: Tran Dinh Loc <dinhlockt02@gmail.com>

core/pkg/sync/file/filepath_sync.go

@@ -171,6 +171,10 @@ func (fs *Sync) fetch(_ context.Context) (string, error) {
 // yamlToJSON is a generic helper function to convert
 // yaml to json
 func yamlToJSON(rawFile []byte) (string, error) {
+	if len(rawFile) == 0 {
+		return "", nil
+	}
+
 	var ms map[string]interface{}
 	// yaml.Unmarshal unmarshals to map[interface]interface{}
 	if err := yaml.Unmarshal(rawFile, &ms); err != nil {

core/pkg/sync/file/filepath_sync_test.go

@@ -309,3 +309,31 @@ func writeToFile(t *testing.T, fetchDirName, fileContents string) {
 		t.Fatal(err)
 	}
 }
+
+func TestFilePathSync_yamlToJSON(t *testing.T) {
+	tests := map[string]struct {
+		input          []byte
+		handleResponse func(t *testing.T, output string, err error)
+	}{
+		"empty": {
+			input: []byte(""),
+			handleResponse: func(t *testing.T, output string, err error) {
+				if err != nil {
+					t.Fatalf("expect no err, got err = %v", err)
+				}
+
+				if output != "" {
+					t.Fatalf("expect output = '', got output = '%v'", output)
+				}
+			},
+		},
+	}
+
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			output, err := yamlToJSON(tt.input)
+
+			tt.handleResponse(t, output, err)
+		})
+	}
+}