fix verify-vendor if go.mod does not contain replace rules

Before this patch, if the go.mod did not contain a replace rule, jq would fail
(Cannot iterate over null), and Bash would produce an error when trying to add
an empty `$key` to `map_requires`:

    make verify-vendor
    + verify-vendor
      jq: error (at <stdin>:581): Cannot iterate over null (null)
      /go/src/github.com/containerd/containerd//script/verify-go-modules.sh: line 44: map_replaces_1[$key]: bad array subscript
      make: *** [Makefile:435: verify-vendor] Error 1

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

script/verify-go-modules.sh

@@ -41,8 +41,8 @@ do
 done<<<$(go mod edit -json | jq -r '.Require[] | .Path +  " # " + .Version')
 while IFS='#' read -r key value
 do
-  map_replaces_1[$key]="$value"
-done<<<$(go mod edit -json | jq -r '.Replace[] | .Old.Path + " # " + .New.Path + " : " + .New.Version')
+  [ "$key" = "" ] || map_replaces_1[$key]="$value"
+done<<<$(go mod edit -json | jq -r 'try .Replace[] | .Old.Path + " # " + .New.Path + " : " + .New.Version')
 popd > /dev/null
 
 # Load the requires and replaces section in the other go.mod file
@@ -51,12 +51,12 @@ declare -A map_replaces_2
 pushd "${ROOT}/$1" > /dev/null
 while IFS='#' read -r key value
 do
-  map_requires_2[$key]="$value"
+  [ "$key" = "" ] || map_requires_2[$key]="$value"
 done<<<$(go mod edit -json | jq -r '.Require[] | .Path +  " # " + .Version')
 while IFS='#' read -r key value
 do
   map_replaces_2[$key]="$value"
-done<<<$(go mod edit -json | jq -r '.Replace[] | .Old.Path + " # " + .New.Path + " : " + .New.Version')
+done<<<$(go mod edit -json | jq -r 'try .Replace[] | .Old.Path + " # " + .New.Path + " : " + .New.Version')
 popd > /dev/null
 
 # signal for errors later