fix phpstan errors

Author: bshaffer

src/JWT.php

@@ -255,6 +255,9 @@ public static function sign(
                     // The last non-empty line is used as the key.
                     $lines = array_filter(explode("\n", $key));
                     $key = base64_decode((string) end($lines));
+                    if (strlen($key) === 0) {
+                        throw new DomainException('Key cannot be empty string');
+                    }
                     return sodium_crypto_sign_detached($msg, $key);
                 } catch (Exception $e) {
                     throw new DomainException($e->getMessage(), 0, $e);
@@ -312,6 +315,12 @@ private static function verify(
                     // The last non-empty line is used as the key.
                     $lines = array_filter(explode("\n", $keyMaterial));
                     $key = base64_decode((string) end($lines));
+                    if (strlen($key) === 0) {
+                        throw new DomainException('Key cannot be empty string');
+                    }
+                    if (strlen($signature) === 0) {
+                        throw new DomainException('Signature cannot be empty string');
+                    }
                     return sodium_crypto_sign_verify_detached($signature, $msg, $key);
                 } catch (Exception $e) {
                     throw new DomainException($e->getMessage(), 0, $e);