use SpotBugs & fix over 300 high priority rule violations (FINERACT-702)

This PR also serves as a base solution for upcoming FINERACT-712 and
FINERACT-696.  It includes using Objects.equals() to prevent NPEs, and
adding gradle check to run SpotBugs on Travis CI.

PS: SpotBugs is the successor to (and same code as) FindBugs.

fineract-provider/build.gradle

@@ -42,6 +42,10 @@ buildscript {
      classpath "org.apache.openjpa:openjpa:$openJPAVersion"
      classpath 'com.radcortez.gradle:openjpa-gradle-plugin:3.1.0'
      classpath 'gradle.plugin.org.nosphere.apache:creadur-rat-gradle:0.2.2'
+      classpath "com.github.spotbugs:spotbugs-gradle-plugin:2.0.1"
+      //Use Guava version 23+ as a workaround to spotbug intergration. 
+      // See: https://github.com/spotbugs/spotbugs-gradle-plugin/issues/128#issuecomment-535864882
+      classpath 'com.google.guava:guava:28.1-jre' 
   }
 }
 
@@ -57,6 +61,7 @@ apply plugin: 'com.bmuschko.tomcat'
 apply plugin: 'project-report'
 apply plugin: 'java-library'
 apply plugin: 'openjpa'
+apply plugin: "com.github.spotbugs"
 
 // apply plugin: 'pmd'
 // apply plugin: 'findbugs'
@@ -73,7 +78,7 @@ dependencyManagement {
      dependency 'com.squareup.okhttp:okhttp:2.0.0'
      dependency 'com.squareup.okhttp:okhttp-urlconnection:2.0.0'
      dependency 'com.google.code.gson:gson:+'
-     dependency 'com.google.guava:guava:15.0'
+     dependency 'com.google.guava:guava:28.1-jre'
      dependency 'joda-time:joda-time:+'
      dependency 'org.apache.commons:commons-email:1.3.3'
      dependency 'org.apache.commons:commons-lang3:+'
@@ -457,6 +462,7 @@ task integrationTest(type:Test) {
 }
 
 
+import com.github.spotbugs.SpotBugsTask
 import groovy.sql.Sql
 
 repositories {
@@ -522,3 +528,10 @@ springBoot {
 bootWar {
     mainClassName = 'org.apache.fineract.ServerWithMariaDB4jApplication'
 }
+
+// To generate an HTML report instead of XML
+tasks.withType(SpotBugsTask) {
+    reports.xml.enabled = false
+    reports.html.enabled = true
+    reportLevel = "high"
+}