Improve handling of null title

[BernieWhite]

Description of the issue

If an empty title is specified using the Title keyword then a null binding exception is generated. The message should be clearer as to what the issue is.

To Reproduce

Steps to reproduce the issue:

Document 'example' {
    $a = $Null;
    Title $a;
}

Expected behaviour

Generate a warning instead specifying that the title is empty and will be skipped.

Error output

Cannot bind argument to parameter 'Text' because it is null.

Module in use and version:

• Module: PSDocs
• Version: 0.7.0

Captured output from $PSVersionTable:

Name                           Value
----                           -----
PSVersion                      7.1.0
PSEdition                      Core
GitCommitId                    7.1.0
OS                             Microsoft Windows 10.0.19042
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0