fix(oidc-provider): resolve getSignedCookie return type (#6346)

Author: bytaesu

packages/better-auth/src/plugins/oidc-provider/index.ts

@@ -408,10 +408,13 @@ export const oidcProvider = (options: OIDCOptions) => {
 
 					if (!consentCode) {
 						// Check for cookie-based consent flow
-						consentCode = await ctx.getSignedCookie(
+						const cookieValue = await ctx.getSignedCookie(
 							"oidc_consent_prompt",
 							ctx.context.secret,
 						);
+						if (cookieValue) {
+							consentCode = cookieValue;
+						}
 					}
 
 					if (!consentCode) {