golang: Make gowork.install optional

Some software releases - e.g. wendelin.core - only use ${go:exe} and
does not put anything into ${gowork:install}.

component/golang/buildout.cfg

@@ -103,7 +103,7 @@ environment-extra =
 #   buildflags = -race
 #
 #
-# Users should add `install` field to [gowork] to describe packages they want to
+# Users can also add `install` field to [gowork] to describe packages they want to
 # be installed (+ automatically their dependencies are installed too). e.g.
 #
 #   [gowork]
@@ -139,6 +139,9 @@ buildflags =
 # empty pkg-config path by default
 cpkgpath =
 
+# by default don't `go install` anything
+install =
+
 # everything is done by dependent parts
 recipe  = plone.recipe.command
 command = :
@@ -166,10 +169,10 @@ update-command = ${:command}
 stop-on-error = true
 
 # install go packages
-# clients should put package list to install to gowork:install ("..." requests installing everything)
+# clients can put package list to install to gowork:install ("..." requests installing everything)
 [gowork.goinstall]
 recipe  = plone.recipe.command
-command = bash -c ". ${gowork:env.sh} && go install ${gowork:buildflags} -v $(echo -n '${gowork:install}' |tr '\n' ' ')"
+command = bash -c ". ${gowork:env.sh} && test -z '${gowork:install}' || go install ${gowork:buildflags} -v $(echo -n '${gowork:install}' |tr '\n' ' ')"
 update-command = ${:command}
 stop-on-error = true