Problem

ServiceConfigUtil.getThrottlePolicy() reads the two required fields of
retryThrottling with JsonUtil.getNumberAsDouble(...).floatValue(). When
either field is missing, getNumberAsDouble returns null and the call fails
with an NPE:

Cannot invoke "java.lang.Double.floatValue()" because the return value of
"io.grpc.internal.JsonUtil.getNumberAsDouble(java.util.Map, String)" is null

ScParser catches it and the channel reports UNKNOWN: failed to parse service config, but the cause doesn't say which field is missing. This was
noted in the code as TODO(dapengzhang0): check if this is null.

Fix

Check both fields with checkNotNull(..., "<field> cannot be empty"), the
same message used for the required retryPolicy and hedgingPolicy fields
in ManagedChannelServiceConfig. The exception type is still the NPE that
ScParser already handles, so the only change is a clearer message.

Testing

Added getThrottlePolicy_failOnMissingMaxTokens and
getThrottlePolicy_failOnMissingTokenRatio to ServiceConfigUtilTest. Both
failed before the fix with the NPE above.