libsecret: retrieve empty password

Since 0ce02e2 (credential/libsecret: store new attributes, 2023-06-16)
a test that stores empty username and password fails when
t0303-credential-external.sh is run with
GIT_TEST_CREDENTIAL_HELPER=libsecret.

Retrieve empty password carefully. This fixes test:

    ok 14 - helper (libsecret) can store empty username

Signed-off-by: M Hickford <mirth.hickford@gmail.com>

contrib/credential/libsecret/git-credential-libsecret.c

@@ -164,6 +164,9 @@ static int keyring_get(struct credential *c)
 			if (g_strv_length(parts) >= 1) {
 				g_free(c->password);
 				c->password = g_strdup(parts[0]);
+			} else {
+				g_free(c->password);
+				c->password = strdup("");
 			}
 			for (int i = 1; i < g_strv_length(parts); i++) {
 				if (g_str_has_prefix(parts[i], "password_expiry_utc=")) {