[webcrypto] Interpret empty BigInteger as 0 according to the spec

LayoutTests/crypto/rsa-oaep-generateKey-parseAlgorithm-failures.html

Index: LayoutTests/crypto/rsa-oaep-generateKey-parseAlgorithm-failures.html
diff --git a/LayoutTests/crypto/rsa-oaep-generateKey-parseAlgorithm-failures.html b/LayoutTests/crypto/rsa-oaep-generateKey-parseAlgorithm-failures.html
index 8e1dd9ed5006432037467e5fcf0aa01ed3f51348..e6a345c5749c4c12c3909edca90a7d4678d9ebe9 100644
--- a/LayoutTests/crypto/rsa-oaep-generateKey-parseAlgorithm-failures.html
+++ b/LayoutTests/crypto/rsa-oaep-generateKey-parseAlgorithm-failures.html
@@ -13,7 +13,7 @@ description("Tests generateKey() with bad RSA-OAEP parameters.");
 jsTestIsAsync = true;
 
 extractable = true;
-keyUsages = ['sign', 'verify'];
+keyUsages = ['encrypt', 'decrypt'];
 
 Promise.resolve(null).then(function() {
     return crypto.subtle.generateKey({name: 'RSA-OAEP', hash: {name: 'sha-1'}, modulusLength: -30}, extractable , keyUsages);