Improve the omit-left-join optimization so that it works in some cases

when the RHS is subject to a UNIQUE but not NOT NULL constraint.

FossilOrigin-Name: 02ba8a7ba7ba71cd7abd5dd3093ea486f53a025f6972bb444f2da37e0e2fc3b2

diff --cc manifest
index 2ab0abc92387e32c79410822896818d7a8386778,4b0592f36ed5b4dcb8bf5164de0e12a7f7a9e578..5ab4a82cba22f29f47ecdd3df8477c8b59930925
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Changes\sto\sthe\sautoconf\starball\sso\sthat\sit\sdoes\snot\stry\sto\suse\ssystem()\swhen\nbuilding\son\siOS.
- D 2018-01-31T01:38:44.477
 -C Fix\sa\sfailing\sassert()\sin\sthe\snew\scode\son\sthis\sbranch.
 -D 2018-01-31T14:07:01.020
++C Improve\sthe\somit-left-join\soptimization\sso\sthat\sit\sworks\sin\ssome\scases\s\nwhen\sthe\sRHS\sis\ssubject\sto\sa\sUNIQUE\sbut\snot\sNOT\sNULL\sconstraint.
++D 2018-01-31T16:50:27.186
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F Makefile.in 7a3f714b4fcf793108042b7b0a5c720b0b310ec84314d61ba7f3f49f27e550ea
@@@ -1704,7 -1702,7 +1704,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P 71e3b715ad36c2132a587b84221be6359c31d7a17c3fba201b7ed3baf17922f1
- R bcc68c8a2653dd62fee16c20bb3144f0
 -P 88411a402b29f19d69e0d06d9099eabb98c238ddb7c07ce110236316f008753e
 -R d723a0554461743d1cc2169a56c72b46
 -U dan
 -Z 54f22eae691d89833b1c2792d257ce0d
++P 32ed9c106710c74a12d60ec33027fd6a9335627c95642ba608593b2735714da6 74d857d178dfadea7e07ba47439fe11aa9d282b54caf78cd6961e593b37406d0
++R ee3ce7babbd6fba601d683cb41b035ba
++T +closed 74d857d178dfadea7e07ba47439fe11aa9d282b54caf78cd6961e593b37406d0
 +U drh
- Z ff1bd702823e7d15c0a453ffcea5582a
++Z 20db364b7139e3a10e7c112d48b03b80

diff --cc manifest.uuid
index 4422b0dd56071b51e3692dd0c564933c5397d7b3,15554a93f998d21669bb1f7be8d4579ad51d7a05..0940797d9ab1cba7ce2e9858b6640677e168a0e5
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 32ed9c106710c74a12d60ec33027fd6a9335627c95642ba608593b2735714da6
 -74d857d178dfadea7e07ba47439fe11aa9d282b54caf78cd6961e593b37406d0
++02ba8a7ba7ba71cd7abd5dd3093ea486f53a025f6972bb444f2da37e0e2fc3b2