Bugfix. Useless auto-fix in AVOID_NULL_CHECKS rule (#848)

### What's done:
* Fixed bug

Co-authored-by: Peter Trifanov <[email protected]>

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter4/NullChecksRule.kt

@@ -83,28 +83,32 @@ class NullChecksRule(configRules: List<RulesConfig>) : DiktatRule(
         }
     }
 
-    @Suppress("UnsafeCallOnNullableType")
+    @Suppress("UnsafeCallOnNullableType", "TOO_LONG_FUNCTION")
     private fun fixNullInIfCondition(condition: ASTNode,
                                      binaryExpression: KtBinaryExpression,
                                      isEqualToNull: Boolean) {
         val variableName = binaryExpression.left!!.text
         val thenCodeLines = condition.extractLinesFromBlock(THEN)
         val elseCodeLines = condition.extractLinesFromBlock(ELSE)
         val text = if (isEqualToNull) {
-            if (elseCodeLines.isNullOrEmpty()) {
-                "$variableName ?: run {\n${thenCodeLines?.joinToString(separator = "\n")}\n}"
-            } else {
-                """
-                    |$variableName?.let {
-                    |${elseCodeLines.joinToString(separator = "\n")}
-                    |}
-                    |?: run {
-                    |${thenCodeLines?.joinToString(separator = "\n")}
-                    |}
-                """.trimMargin()
+            when {
+                elseCodeLines.isNullOrEmpty() -> "$variableName ?: run {\n${thenCodeLines?.joinToString(separator = "\n")}\n}"
+                thenCodeLines!!.singleOrNull() == "null" -> """
+                        |$variableName?.let {
+                        |${elseCodeLines.joinToString(separator = "\n")}
+                        |}
+                    """.trimMargin()
+                else -> """
+                        |$variableName?.let {
+                        |${elseCodeLines.joinToString(separator = "\n")}
+                        |}
+                        |?: run {
+                        |${thenCodeLines.joinToString(separator = "\n")}
+                        |}
+                    """.trimMargin()
             }
         } else {
-            if (elseCodeLines.isNullOrEmpty()) {
+            if (elseCodeLines.isNullOrEmpty() || (elseCodeLines.singleOrNull() == "null")) {
                 "$variableName?.let {\n${thenCodeLines?.joinToString(separator = "\n")}\n}"
             } else {
                 """

diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckExpected.kt

@@ -21,13 +21,29 @@ print("qwe")
 }
 ?: run {
 print("asd")
+}
+
+    some?.let {
+print("qweqwe")
 }
 
     some?.let {
 print("qqq")
 }
 ?: run {
 print("www")
+}
+
+    some?.let {
+print("ttt")
+}
+
+    some?.let {
+print("ttt")
+}
+?: run {
+null
+value
 }
 }
 

diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckTest.kt

@@ -22,10 +22,29 @@ fun test() {
         print("qwe")
     }
 
+    if (some == null) {
+        null
+    } else {
+        print("qweqwe")
+    }
+
     if (some != null) {
         print("qqq")
     } else {
         print("www")
     }
+
+    if (some != null) {
+        print("ttt")
+    } else {
+        null
+    }
+
+    if (some != null) {
+        print("ttt")
+    } else {
+        null
+        value
+    }
 }