fixup for Optic concat expression test

[ehennum]

Because the enode Optic API is no longer sniffing for and enforcing data types but, instead, allowing implicit casts to occur, a string numeric in the concat test is turning into a numeric, which is causing an equality test to fail.

[ehennum]

Assigning for awareness and not because additional coverage is needed.

The API didn't change, only a unit test.

The error in the "expressions concat" unit test should be gone now.

If so, this issue can be closed.

[georgeajit]

No regressions seen. Verified on develop branch with 10.0.1 ML server build dated 07022019.