Expose property name when checking key components

Expose property name when checking key components

Experience with null enforcement of key components indicates we should
carry the name of the property in the exception message -- otherwise
the string is not really helpful.

This patch re-organizes the checking so that we do that, via
newly-introduced CodeHelpers.requireKeyProp() method.

Since we are in the area, also codegen methods' handling of arrays by
creating an explicit 'cloneCall()' utility method.

JIRA: MDSAL-599
Change-Id: If9a87b4976ecdcd09e1fdd83ddfaf03ab4d09a85
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>