Make sure the group_concat() function returns an empty string, not a NULL,

if it has at least one input row. Fix for ticket [55746f9e65f8587].

FossilOrigin-Name: d01cedaa73d8f9e5502502a1068a9509d1de295c

diff --cc manifest
index 28652fe3ac9f05577905d48a3e5e5cb67a614cbb,c8a928d23071f6f0c4818a9c39aa9a9893297ede..77f24441e531a6ec80df4d9e69f2737bf5068eeb
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Add\sthe\sSQLITE_IOCAP_IMMUTABLE\sbit\sas\sa\spossible\sreturn\svalue\sfrom\nthe\sxDeviceCharacteristics\smethod\sin\sthe\sVFS.\s\sAdd\sthe\s"nolock"\sand\n"immutable"\squery\sparameters\sto\sURI\sfilenames.
- D 2014-05-07T15:46:04.082
 -C Ensure\sthat\sthe\ssqlite3StrAccumAppend()\sroutine\sis\snever\scalled\swith\sa\sNULL\nsecond\sargument.\s\sDoing\sso\sis\sharmless\swhen\sN==0,\sbut\sit\scauses\san\sassert()\nto\sfail\sthat\swas\splaced\sto\squiet\sstatic\sanalyzers.
 -D 2014-05-07T18:21:56.753
++C Make\ssure\sthe\sgroup_concat()\sfunction\sreturns\san\sempty\sstring,\snot\sa\sNULL,\s\nif\sit\shas\sat\sleast\sone\sinput\srow.\sFix\sfor\sticket\s[55746f9e65f8587].
++D 2014-05-07T18:23:04.692
  F Makefile.arm-wince-mingw32ce-gcc d6df77f1f48d690bd73162294bbba7f59507c72f
  F Makefile.in dd2b1aba364ff9b05de41086f74407f285c57670
  F Makefile.linux-gcc 91d710bdc4998cb015f39edf3cb314ec4f4d7e23
@@@ -1169,8 -1169,7 +1169,8 @@@ F tool/vdbe_profile.tcl 67746953071a9f8
  F tool/warnings-clang.sh f6aa929dc20ef1f856af04a730772f59283631d4
  F tool/warnings.sh d1a6de74685f360ab718efda6265994b99bbea01
  F tool/win/sqlite.vsix a94fb9b1b1ef06efc2898975cdfcfa9643731f5e
- P 99d96765cc378fde7b285f4577ea2b5d130d9a61 e193aced2942e7405d0f45f90d4954b5663b4ba5
- R 397fc32d1bd36ead839d4c78d231487f
- T +closed e193aced2942e7405d0f45f90d4954b5663b4ba5
 -P 0deac8737545a020d344be96fff16660a7977ab8
++P 1a0d7d3d9dd54b783e3a805961287dd01f94770c f03fbf3700d9d4a654e3aa2e5caa810a8416bed9
+ R aaf6cc2f3f5ec9bee45f0a384deaafae
++T +closed f03fbf3700d9d4a654e3aa2e5caa810a8416bed9
  U drh
- Z 57c6364c482e6b92f356dd08394fdb62
 -Z daf5890128d88bd8d8d454e0919fc158
++Z fad451fce86c7077ae7b04e526f1422c

diff --cc manifest.uuid
index 9da16245c08048e5eab966d8f9ae581d4c84f045,a75992b9d9de7ff0d2b61645d6c3e06cf3aa258a..ff91e39b9fc155467d80c4103654ea9b1197541c
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 1a0d7d3d9dd54b783e3a805961287dd01f94770c
 -f03fbf3700d9d4a654e3aa2e5caa810a8416bed9
++d01cedaa73d8f9e5502502a1068a9509d1de295c