Release 2.0.0: interpolating a NULL propagates it

[jdatcmd]

Third of three. Ships the behaviour change from #8 and #9.

Why major

One behaviour change, no catalog change: interpolating a NULL propagates it instead of rendering an empty string.

-- return "#{country}-#{region}-#{code}" with a NULL region
--   before: 'US--123'   a string that inserts, indexes and joins like real data
--   now:    NULL

Affects plxruby, plxphp, plxjs, plxts, plxpython3, plxgo. plxplsql and plxtsql already propagated. plxcobol is unchanged (the plx_strbuild accumulator's non-strict append is deliberate and separately tested). A RAISE message keeps its empty-string fallback so one NULL can't swallow a diagnostic.

The upgrade is not as alarming as it sounds

ALTER EXTENSION plx UPDATE TO '2.0.0' does not change any function that already exists. plx transpiles at CREATE FUNCTION time and stores the result in pg_proc.prosrc, so a catalogued function keeps the plpgsql it was created with. The new behaviour arrives when its DDL is next run.

Verified rather than assumed — built 1.3.2, created a function, installed 2.0.0:

created under 1.3.2              'user  has 3 items'
existing function after upgrade  'user  has 3 items'   <- unchanged
after recreate                   NULL                  <- new behaviour

So the risk window is a redeployment, not the upgrade. The CHANGELOG carries a query to find the affected functions ahead of that, tested with a positive and a negative control:

flagged_as_affected
-------------------
 aff_value(text)        <- value-position interpolation
                        <- aff_msgonly(text) correctly NOT flagged

The negative control matters: a function whose only COALESCE is in a RAISE message is not affected, and a naive prosrc LIKE '%COALESCE%' would wrongly flag it.

Mechanics

• plx.control default_version and META.json to 2.0.0
• plx--2.0.0.sql, identical to 1.3.2 since no SQL object changes
• plx--1.3.2--2.0.0.sql, version-advancing, with a comment explaining that catalogued functions are unaffected
• CHANGELOG [2.0.0] with the change, the reasoning, and the upgrade query

Verification on PG18.4

Clean build, 13/13 installcheck, differentialcheck 554 matching, 17 documented, 0 unexplained, 0 stale.

before          1.3.2
after_upgrade   2.0.0
ok              'user bob has 3 items'
null_propagates NULL
fresh_default   2.0.0

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