PBKDF2 allows zero length and returns and empty string

This CL adapts our implementation to the spec change described in the
PR#380 [1]. Instead of trowing an OperationError exception, we allow now
zero as value for the 'length' parameter.

Given that the deriveBits operation must result in an empty string, this
change early returns to avoid the unnecessary computation of the bits
derivation.

The specific WTP defined for this case are modified in this CL as well,
so no additional test cases are needed.

[1] w3c/webcrypto#380

Bug: 376493194
Change-Id: If685c349a0a9d134a8e8f7c902e8aac342945226

Author: javifernandez

WebCryptoAPI/derive_bits_keys/derived_bits_length_testcases.js

@@ -3,7 +3,7 @@ var testCases = {
         {length: 256, expected: algorithms["HKDF"].derivation},
         {length: 384, expected: algorithms["HKDF"].derivation384},
         {length: 230, expected: undefined}, // should throw an exception, not multiple of 8
-        {length: 0, expected: undefined}, // explicitly disallowed, so should throw
+        {length: 0, expected: emptyArray},
         {length: null, expected: undefined }, // should throw an exception
         {length: undefined, expected: undefined }, // should throw an exception
         {length: "omitted", expected: undefined }, // default value is null, so should throw
@@ -12,7 +12,7 @@ var testCases = {
         {length: 256, expected: algorithms["PBKDF2"].derivation},
         {length: 384, expected: algorithms["PBKDF2"].derivation384},
         {length: 230, expected: undefined}, // should throw an exception, not multiple of 8
-        {length: 0, expected: undefined}, // explicitly disallowed, so should throw
+        {length: 0, expected: emptyArray},
         {length: null, expected: undefined }, // should throw an exception
         {length: undefined, expected: undefined }, // should throw an exception
         {length: "omitted", expected: undefined }, // default value is null, so should throw

WebCryptoAPI/derive_bits_keys/pbkdf2.js

@@ -103,16 +103,6 @@ function define_tests() {
 
                         });
 
-                        // 0 length (OperationError)
-                        subsetTest(promise_test, function(test) {
-                            return subtle.deriveBits({name: "PBKDF2", salt: salts[saltSize], hash: hashName, iterations: parseInt(iterations)}, baseKeys[passwordSize], 0)
-                            .then(function(derivation) {
-                                assert_unreached("0 length should have thrown an OperationError");
-                            }, function(err) {
-                                assert_equals(err.name, "OperationError", "deriveBits with 0 length correctly threw OperationError: " + err.message);
-                            });
-                        }, testName + " with 0 length");
-
                         // length not multiple of 8 (OperationError)
                         subsetTest(promise_test, function(test) {
                             return subtle.deriveBits({name: "PBKDF2", salt: salts[saltSize], hash: hashName, iterations: parseInt(iterations)}, baseKeys[passwordSize], 44)