assert_output without args asserts output is non-empty

src/assert.bash

diff --git a/src/assert.bash b/src/assert.bash
index 5dba42c..b680fad 100644
--- a/src/assert.bash
+++ b/src/assert.bash
@@ -185,9 +185,14 @@
 assert_output() {
   local -i is_mode_partial=0
   local -i is_mode_regexp=0
+  local -i is_mode_nonempty=0
   local -i use_stdin=0
 
   # Handle options.
+  if (( $# == 0 )); then
+    is_mode_nonempty=1
+  fi
+
   while (( $# > 0 )); do
     case "$1" in
       -p|--partial) is_mode_partial=1; shift ;;
@@ -214,7 +219,13 @@
   fi
 
   # Matching.
-  if (( is_mode_regexp )); then
+  if (( is_mode_nonempty )); then
+    if [ -z "$output" ]; then
+      echo 'expected non-empty output, but output was empty' \
+      | batslib_decorate 'no output' \
+      | fail
+    fi
+  elif (( is_mode_regexp )); then
     if [[ '' =~ $expected ]] || (( $? == 2 )); then
       echo "Invalid extended regular expression: \`$expected'" \
         | batslib_decorate 'ERROR: assert_output' \

test/50-assert-15-assert_output.bats

diff --git a/test/50-assert-15-assert_output.bats b/test/50-assert-15-assert_output.bats
index 7efee71..16ced75 100755
--- a/test/50-assert-15-assert_output.bats
+++ b/test/50-assert-15-assert_output.bats
@@ -26,6 +26,25 @@
   [ "${lines[3]}" == '--' ]
 }
 
+@test 'assert_output(): succeeds if output is non-empty' {
+  run echo 'a'
+  run assert_output
+echo "$output"
+  [ "$status" -eq 0 ]
+  [ "${#lines[@]}" -eq 0 ]
+}
+
+@test 'assert_output(): fails if output is empty' {
+  run echo ''
+  run assert_output
+echo "$output"
+  [ "$status" -eq 1 ]
+  [ "${#lines[@]}" -eq 3 ]
+  [ "${lines[0]}" == '-- no output --' ]
+  [ "${lines[1]}" == 'expected non-empty output, but output was empty' ]
+  [ "${lines[2]}" == '--' ]
+}
+
 @test 'assert_output() - : reads <expected> from STDIN' {
   run echo 'a'
   run assert_output - <<STDIN