Allow nil to truncate template function (#5656)

Fixes #5643

pkg/export/template.go

@@ -56,7 +56,15 @@ func (t *Template) parseTemplate(tpl string) (*template.Template, error) {
 		"join":        templateJoin,
 		"tablerow":    t.tableRow,
 		"tablerender": t.tableRender,
-		"truncate":    text.Truncate,
+		"truncate": func(maxWidth int, v interface{}) (string, error) {
+			if v == nil {
+				return "", nil
+			}
+			if s, ok := v.(string); ok {
+				return text.Truncate(maxWidth, s), nil
+			}
+			return "", fmt.Errorf("invalid value; expected string, got %T", v)
+		},
 	}
 
 	if !t.io.ColorEnabled() {

pkg/export/template_test.go

@@ -263,6 +263,38 @@ func Test_executeTemplate(t *testing.T) {
 			},
 			wantW: "This is a ...",
 		},
+		{
+			name: "truncate with JSON null",
+			args: args{
+				json:     strings.NewReader(`{}`),
+				template: `{{ truncate 13 .title }}`,
+			},
+			wantW: "",
+		},
+		{
+			name: "truncate with piped JSON null",
+			args: args{
+				json:     strings.NewReader(`{}`),
+				template: `{{ .title | truncate 13 }}`,
+			},
+			wantW: "",
+		},
+		{
+			name: "truncate with piped JSON null in parenthetical",
+			args: args{
+				json:     strings.NewReader(`{}`),
+				template: `{{ (.title | truncate 13) }}`,
+			},
+			wantW: "",
+		},
+		{
+			name: "truncate invalid type",
+			args: args{
+				json:     strings.NewReader(`{"title": 42}`),
+				template: `{{ (.title | truncate 13) }}`,
+			},
+			wantErr: true,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {