Annotate API reference types for nullable awareness

[pleonex]

Description

Introduced with C# 8.0, we can annotate the code to specify if a reference-type variable can be null or not. Then, we can enable a new static analysis check that check for potential code paths that will de-reference a null pointer, throwing a NullReferenceException.
Annotating the API allows user to enable the feature as well and ensure a safe usage of the code. Fix all the related warnings

This is introducing the following breaking changes:

• TextDataReader will not return null string if it starts reading from the end already. It will throw EndOfStreamException.
• Po initializes an empty header.
• Properties of PoHeader are initialized to an empty string instead of null values, CreationDate is set to current time.
• The setter of Po.Header will not check if the header has some required non-empty properties.
• Po2Binary will write the PO header only if ProjectIdVersion is non-empty instead of checking if the Header property is null.

Example

To enable the feature in any user project, just set <Nullable>enable</Nullable> in the csproj.