Zero-length environment variables reported as null

[tiymat]

Describe the bug: is-null reports environment variables set to zero-length strings as null, inconsistent with its treatment of local and global variables.

Expected behaviour: Environment variables set to the empty string should be treated by is-null as non-null.

Screenshots:

~ » set x=""
~ » global y=""
~ » export z=""
~ » is-null x y z
x: defined and not null
y: defined and not null
z: undefined or null

Platform (please complete the following information):

• OS, output from uname -a if supported: Ubuntu 23.04
• Terminal Emulator: GNOME Terminal
• Murex version, output from version --no-app-name: 5.4.1000

[lmorg]

Another good spot. Just pushed a fix up to develop