if subfilter is not detected return instead of throw

__tests__/index.test.ts

@@ -13,7 +13,7 @@ describe("PDF parser", () => {
     const parser = new PDFparser(file);
     const actual = await parser.getPDFMeta();
 
-    expect(actual.pages).toEqual(1);
+    expect(actual.signatures.length).toEqual(1);
   });
 
   it("should return pdf document metadata without signatures", async () => {

src/lib/index.ts

@@ -7,7 +7,8 @@ export const verifyPDF = (pdf: Buffer) => {
   try {
     checkForSubFilter(pdfBuffer);
   } catch (error) {
-    throw new Error(error.message);
+    //SubFilter is only available if there is a sig in the pdf
+    return null;
   }
 
   try {