Fix empty updates in dynamic init scripts

opam already ensured that empty segments are never added to environment
variables with opam env, but the filter was not there for writing the
dynamic init scripts.

master_changes.md

@@ -57,6 +57,7 @@ users)
 ## Clean
 
 ## Env
+  * Don't write empty environment update segments to variables.sh (`FOO += ""` no longer adds `FOO='':"$FOO"; export FOO;`) [#6198 @dra27]
 
 ## Opamfile
 

src/state/opamEnv.ml

@@ -1184,7 +1184,16 @@ let write_custom_init_scripts root custom =
     ) custom
 
 let write_dynamic_init_scripts st =
-  let updates = updates ~set_opamroot:false ~set_opamswitch:false st in
+  (* Empty environment updates must not be written to the scripts *)
+  let is_not_empty_update = function
+  | {envu_value = ""; envu_op; _} ->
+    envu_op = Eq
+  | _ -> true
+  in
+  let updates =
+    updates ~set_opamroot:false ~set_opamswitch:false st
+    |> List.filter is_not_empty_update
+  in
   try
     if OpamStateConfig.is_newer_than_self
         ~lock_kind:`Lock_write st.switch_global then