BaseObject#set throws a NullPointerException when setting an existing password property to the form placeholder

Steps to reproduce

• Have an XClass with a Password property, and an object of that class where the password is already set
• Call $obj.set('password', '********') (e.g. the placeholder value sent back by a form displaying the password field in edit mode)

Expected

The existing password value is kept, as it was before 17.9.0RC1.

Actual

java.lang.NullPointerException: Cannot invoke "com.xpn.xwiki.objects.BaseProperty.getValue()" because "newProp" is null
 at com.xpn.xwiki.objects.BaseObject.set(BaseObject.java:522)
 at com.xpn.xwiki.api.Object.set(Object.java:206)

Analysis

PasswordClass#fromString deliberately returns null when given the form password placeholder, so that the stored password is not replaced by it. Before XWIKI-21417, BaseObject#set assigned the result of fromString to the property and skipped it when null, so the existing value was kept. Since XWIKI-21417, BaseObject#set updates the existing property in place with prop.setValue(newProp.getValue()), without checking whether newProp is null.

The null returned by fromString should be handled as "keep the current value".

This breaks for example the JIRA Macro basic auth configuration as soon as a second entry is saved: JIRA-126.