Fix tests and use proper types for disregarding XMLComponent

src/file/table/table-cell-margin.ts

@@ -21,8 +21,10 @@ export class TableCellMargin extends XmlComponent {
         super("w:tblCellMar");
     }
 
-    public prepForXml(): IXmlableObject {
-        return this.root.length > 0 ? super.prepForXml() : "";
+    public prepForXml(): IXmlableObject | undefined {
+        if (this.root.length > 0) {
+            return super.prepForXml();
+        }
     }
 
     public addTopMargin(value: number, type: WidthType = WidthType.DXA): void {

src/file/table/table-cell.spec.ts

@@ -8,8 +8,7 @@ describe("TableCellBorders", () => {
     describe("#prepForXml", () => {
         it("should not add empty borders element if there are no borders defined", () => {
             const tb = new TableCellBorders();
-            const tree = new Formatter().format(tb);
-            expect(tree).to.deep.equal("");
+            expect(() => new Formatter().format(tb)).to.throw();
         });
     });
 

src/file/table/table-cell.ts

@@ -29,8 +29,10 @@ export class TableCellBorders extends XmlComponent {
         super("w:tcBorders");
     }
 
-    public prepForXml(): IXmlableObject {
-        return this.root.length > 0 ? super.prepForXml() : "";
+    public prepForXml(): IXmlableObject | undefined {
+        if (this.root.length > 0) {
+            return super.prepForXml();
+        }
     }
 
     public addTopBorder(style: BorderStyle, size: number, color: string): TableCellBorders {

src/file/table/table.ts

@@ -126,9 +126,13 @@ export class TableCell extends XmlComponent {
         return this;
     }
 
-    public prepForXml(): IXmlableObject {
+    public prepForXml(): IXmlableObject | undefined {
         // Cells must end with a paragraph
         const retval = super.prepForXml();
+        if (!retval) {
+            return undefined;
+        }
+
         const content = retval["w:tc"];
         if (!content[content.length - 1]["w:p"]) {
             content.push(new Paragraph().prepForXml());