[build] Enable spotbugs (#3601)

Benign spotbugs warnings were suppressed, and all others were fixed. Bug
descriptions are documented here:
https://spotbugs.readthedocs.io/en/stable/bugDescriptions.html

Co-authored-by: Austin Shalit <austinshalit@gmail.com>

wpiutil/src/main/java/edu/wpi/first/util/CombinedRuntimeLoader.java

@@ -100,7 +100,11 @@ public final class CombinedRuntimeLoader {
         if (outputFile.toFile().exists()) {
           continue;
         }
-        outputFile.getParent().toFile().mkdirs();
+        var parent = outputFile.getParent();
+        if (parent == null) {
+          throw new IOException("Output file has no parent");
+        }
+        parent.toFile().mkdirs();
 
         try (var os = Files.newOutputStream(outputFile)) {
           int readBytes;