Exception loading an older model file with no strings

[eerhardt]

When I changed our model format for the ComponentCatalog changes (#970), I refactored some model loading code incorrectly. There are some existing models that are breaking our validation checks:

The old code was doing:

machinelearning/src/Microsoft.ML.Data/Model/ModelHeader.cs

Lines 419 to 424 in 655c2e2

	if (header.FpStringTable == 0)
	{
	// No strings.
	strings = null;
	ex = null;
	return true;

The new code now does:

machinelearning/src/Microsoft.ML.Data/Model/ModelHeader.cs

Lines 482 to 486 in a44e131

	if (header.FpStringTable == 0)
	{
	// No strings.
	strings = null;
	}

Notice that it doesn't return early in the new code. Lower in the method there are checks here:

machinelearning/src/Microsoft.ML.Data/Model/ModelHeader.cs

Lines 538 to 541 in a44e131

	Contracts.CheckDecode(header.FpTail == reader.FpCur() - fpMin);
	ulong tail = reader.ReadUInt64();
	Contracts.CheckDecode(tail == TailSignatureValue, "Corrupt model file tail");

These checks are now failing with older model files. They were never run in the old code when there were no strings, so we shouldn't be running them anymore when reading older model files.

/cc @yaeldekel