canvas_update script: fix bug with missing gems/plugins and/or

vendor/plugins

When either gems/plugins or vendor/plugins doesn't exist in the repo,
the script would print an error as it tried to list the directory
contents through globbing.  This fixes that error.

Fixes CNVS-19519

Test Plan:
    - Create a new checkout with no vendor/plugins directory, or delete
      the existing one (not recommended)
    - Run the script and ensure it does not report an error with
      a missing vendor/plugins/*
    - Ensure the script completes without error

Change-Id: I4d49fc090d4abc21ac8e8005f4ed83c47b4e89c5
Reviewed-on: https://gerrit.instructure.com/50961
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins
QA-Review: Benjamin Porter <bporter@instructure.com>

script/canvas_update

@@ -38,20 +38,26 @@ function intro_message {
   echo "-----------------------------" >>"$LOG"
 }
 
+function update_plugin {
+  (
+    cd "$1"
+    if is_git_dir; then
+      echo_console_and_log "  Updating plugin $1 ..."
+      ( git checkout master >>"$LOG" 2>&1 && \
+        git pull --rebase >>"$LOG" 2>&1 ) || \
+      ( echo "  failed to pull plugin (see $LOG)"; kill -INT $$ )
+    fi
+  )
+}
+
 function update_plugins {
   # Loop through each plugin dir, and if it's a git repo, update it
   # This needs to be done first so that db:migrate can pull in any plugin-
   # precipitated changes to the database.
-  for PLUGIN in {gems,vendor}/plugins/*; do
-    (
-      cd "$PLUGIN"
-      if is_git_dir; then
-        echo_console_and_log "  Updating plugin $PLUGIN ..."
-        ( git checkout master >>"$LOG" 2>&1 && \
-          git pull --rebase >>"$LOG" 2>&1 ) || \
-        ( echo "  failed to pull plugin (see $LOG)"; kill -INT $$ )
-      fi
-    )
+  for dir in {gems,vendor}; do
+    if [ -d "$dir/plugins" ]; then
+      for plugin in $dir/plugins/*; do update_plugin "$plugin"; done
+    fi
   done
 }