Don't check `$1` unless `$#` is >= 1.

Fixes #873. Partially fixes #868.

nvm.sh

@@ -2239,7 +2239,7 @@ $NVM_LS_REMOTE_POST_MERGED_OUTPUT" | command grep -v "N/A" | command sed '/^$/d'
 }
 
 nvm_supports_source_options() {
-  [ "_$(echo 'echo $1' | . /dev/stdin yes 2> /dev/null)" = "_yes" ]
+  [ "_$(echo '[ $# -gt 0 ] && echo $1' | . /dev/stdin yes 2> /dev/null)" = "_yes" ]
 }
 
 nvm_supports_xz() {