[Fix] nvm_ls: no blank line before system when nothing is installed

`nvm ls` then printed an empty row ahead of the system version.

nvm.sh

@@ -2193,11 +2193,11 @@ nvm_ls() {
     case "${PATTERN}" in
       '' | v)
         if [ -n "${SYSTEM_VERSION}" ]; then
-          VERSIONS="${VERSIONS}
-system ${SYSTEM_VERSION}"
+          VERSIONS="${VERSIONS:+${VERSIONS}
+}system ${SYSTEM_VERSION}"
         else
-          VERSIONS="${VERSIONS}
-system"
+          VERSIONS="${VERSIONS:+${VERSIONS}
+}system"
         fi
       ;;
       system)

test/fast/Unit tests/nvm_ls with only a system version

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+ORIG_NVM_DIR="${NVM_DIR}"
+TMP_DIR=''
+
+cleanup () {
+  case "${TMP_DIR}" in
+    /*) rm -rf "${TMP_DIR}" ;;
+  esac
+  NVM_DIR="${ORIG_NVM_DIR}"
+  export NVM_DIR
+}
+
+die () { echo "$@" ; cleanup ; exit 1; }
+
+: nvm.sh
+\. ../../../nvm.sh
+
+TMP_DIR="$(mktemp -d)"
+NVM_DIR="${TMP_DIR}/nvm"
+export NVM_DIR
+mkdir -p "${NVM_DIR}" || die 'could not create NVM_DIR'
+
+nvm_has_system_node() { return 0; }
+nvm_has_system_iojs() { return 1; }
+node() { echo 'v20.0.0'; }
+
+OUTPUT="$(nvm_ls)"
+[ "_${OUTPUT}" = '_system v20.0.0' ] || die "nvm_ls with nothing installed should print only the system version; got >${OUTPUT}<"
+
+cleanup