[confmap/envprovider] Fix empty default resolving to nil instead of empty string

.chloggen/fix-envprovider-empty-default.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. receiver/otlp)
+component: provider/env
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix empty env var default resolving to nil instead of empty string
+
+# One or more tracking issues or pull requests related to the change
+issues: [14587]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  When using ${env:VAR:-} with an unset variable, the empty default now correctly
+  resolves to an empty string instead of nil.
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

confmap/provider/envprovider/provider.go

@@ -64,6 +64,10 @@ func (emp *provider) Retrieve(_ context.Context, uri string, _ confmap.WatcherFu
 		emp.logger.Info("Configuration references empty environment variable", zap.String("name", envVarName))
 	}
 
+	if val == "" && !exists && defaultValuePtr != nil {
+		return confmap.NewRetrieved(val)
+	}
+
 	return confmap.NewRetrievedFromYAML([]byte(val))
 }
 

confmap/provider/envprovider/provider_test.go

@@ -190,6 +190,17 @@ func TestEnvWithDefaultValue(t *testing.T) {
 	assert.NoError(t, env.Shutdown(context.Background()))
 }
 
+func TestEmptyDefaultReturnsEmptyString(t *testing.T) {
+	env := createProvider()
+	ret, err := env.Retrieve(context.Background(), "env:MY_VAR:-", nil)
+	require.NoError(t, err)
+
+	raw, err := ret.AsRaw()
+	require.NoError(t, err)
+	require.IsType(t, "", raw, "empty default should resolve to empty string, not nil")
+	assert.Empty(t, raw)
+}
+
 func createProvider() confmap.Provider {
 	return NewFactory().Create(confmaptest.NewNopProviderSettings())
 }