fix: handle undefined signers in getAllFiles filter

Add optional chaining to prevent TypeError when value.signers
is undefined in file filtering by signer_uuid. This resolves
the error 'can't access property "filter", value.signers is
undefined' that occurred when navigating to sign PDF page.

The issue occurred when files without initialized signers array
were being filtered, causing the application to crash on the
SignPDF view.

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

src/store/files.js

@@ -278,7 +278,7 @@ export const useFilesStore = function(...args) {
 							Object.entries(this.files).filter(([key, value]) => {
 								if (filter.signer_uuid) {
 									// return true when found signer by signer_uuid
-									return value.signers.filter((signer) => {
+									return value.signers?.filter((signer) => {
 										// filter signers by signer_uuid
 										return signer.sign_uuid === filter.signer_uuid
 									}).length > 0