Fixed a bug in custom rules that only check for errors and don't retu…

…rn any fixes. Follow-on to #46.

src/main/java/com/diffplug/gradle/spotless/Formatter.java

@@ -113,9 +113,14 @@ String applySteps(String unix, File file) throws Error {
 		for (FormatterStep step : steps) {
 			try {
 				String formatted = step.format(unix, file);
-				// should already be unix-only, but
-				// some steps might misbehave
-				unix = LineEnding.toUnix(formatted);
+				if (formatted == null) {
+					// This probably means it was a step that only checks
+					// for errors and doesn't actually have any fixes.
+					// No exception was thrown so we can just continue.
+				} else {
+					// Should already be unix-only, but some steps might misbehave.
+					unix = LineEnding.toUnix(formatted);
+				}
 			} catch (Error e) {
 				logger.error("Step '" + step.getName() + "' found problem in '" + projectDirectory.relativize(file.toPath()) + "':\n" + e.getMessage());
 				throw e;

src/test/java/com/diffplug/gradle/spotless/ErrorShouldRethrow.java

@@ -66,4 +66,25 @@ public void noSwearing() throws Exception {
 		String actualStart = actualLines.subList(0, numNewlines + 1).stream().collect(Collectors.joining("\n"));
 		Assert.assertEquals(expectedToStartWith, actualStart);
 	}
+
+	@Test
+	public void noSwearingPassesIfNoSwears() throws Exception {
+		write("build.gradle",
+				"plugins {",
+				"    id 'com.diffplug.gradle.spotless'",
+				"}",
+				"spotless {",
+				"    format 'misc', {",
+				"        lineEndings 'UNIX'",
+				"        target file('README.md')",
+				"        custom 'no swearing', {",
+				"             if (it.toLowerCase().contains('fubar')) {",
+				"                 throw new AssertionError('No swearing!');",
+				"             }",
+				"        }",
+				"    }",
+				"}");
+		write("README.md", "This code is fun.");
+		gradleRunner().withArguments("spotlessCheck").build();
+	}
 }