Avoid encoding empty titlePg values

[tyler-sommer]

We ran into an issue where taking a human-crafted word doc and modifying it with godocx would then lead to godocx not being able to parse the document, citing:

failed to open document: invalid OnOff string

I tracked this back to the TitlePG field on ctypes.SectionProp. Prior to modification by godocx, this was encoded as:

<w:titlePg/>

After modification with godocx, this would be encoded as:

<w:titlePg w:val=""></w:titlePg>

The empty string in Val would cause decoding to fail.

This PR fixes the issue, however, I am not sure this is the fully correct way to go.

Many *OnOff fields are just that (pointer to OnOff), but TitlePG is *GenSingleStrVal[stypes.OnOff] and GenSingleStrVal encodes regardless of whether the value it stores is empty. Rather than modifying GenSingleStrVal, which would probably break things, I opted to just not encode TitlePG when it is empty. Let me know if there's a better way to handle this.

Thanks!

[tyler-sommer]

After some more testing, this does not work as expected (completely breaks page title rendering, it seems). I'll dig into this some more. In the meantime, would love any suggestions :)

Thanks again!