issues with Strings being compared with either == or !=

The Static Code Analyzer Spot Bugs found the following issues with Strings being compared with either == or != and should be using **equals()**.

**From Spot Bugs:**

Comparison of String objects using == or !=

(https://spotbugs.readthedocs.io/en/latest/bugDescriptions.html#ES_COMPARING_STRINGS_WITH_EQ)
This code compares java.lang.String objects for reference equality using the == or != operators. Unless both strings are either
constants in a source file, or have been interned using the String.intern() method, the same string value may be represented by two
different String objects. Consider using the **equals(Object)** method instead.

Bug kind and pattern: ES - ES_COMPARING_STRINGS_WITH_EQ

**Occurrences:**

https://github.com/imagej/ImageJ/blob/master/ij/gui/PlotContentsDialog.java

if (allTableNames[i] == previousTableName)

https://github.com/imagej/ImageJ/blob/master/ij/plugin/SimpleCommands.java

if (label2!=label) {

https://github.com/imagej/ImageJ/blob/master/ij/macro/FunctionFinder.java

if (exists=="0") {

https://github.com/imagej/ImageJ/blob/master/ij/plugin/frame/Recorder.java

if (commandOptions==null || commandOptions==" ")

occurs twice

Review the reported comparisons in ij/gui/PlotContentsDialog.java, ij/plugin/SimpleCommands.java, ij/macro/FunctionFinder.java, and ij/plugin/frame/Recorder.java. Start with the listed SpotBugs ES_COMPARING_STRINGS_WITH_EQ warning and verify each occurrence's intended string-value comparison. Done means the affected comparisons are corrected and the SpotBugs warning no longer reports these occurrences.

Written by the indexing model from the issue text.