Fix a bug with Cancelable#cancel(String)

The various implementations of this method (which are all copy-pasted
from one another, due to Java's single inheritance limitations) all
suffered from the same bug: calling `cancel(null)` would not actually
cancel the operation. Actually, this was the intended design (so you
could potentially "uncancel" an operation), but I decided that:

1.  Uncanceling is not something anyone should need to do;
2.  It violates the PoLA for `cancel(null)` not to cancel things;
3.  The ModuleRunner calls `cancel(e.getMessage())` from a caught
    ModuleException, which wouldn't actually cancel the module execution
    if the exception had a null message, which it did thanks to the
    AbstractInputHarvester doing `throw new ModuleCanceledException()`.

Author: ctrueden

src/main/java/org/scijava/command/ContextCommand.java

@@ -96,7 +96,7 @@ public boolean isCanceled() {
 	/** Cancels the command execution, with the given reason for doing so. */
 	@Override
 	public void cancel(final String reason) {
-		cancelReason = reason;
+		cancelReason = reason == null ? "" : reason;
 	}
 
 	@Override

src/main/java/org/scijava/command/DynamicCommand.java

@@ -131,7 +131,7 @@ public boolean isCanceled() {
 
 	@Override
 	public void cancel(final String reason) {
-		cancelReason = reason;
+		cancelReason = reason == null ? "" : reason;
 	}
 
 	@Override

src/main/java/org/scijava/command/ModuleCommand.java

@@ -92,8 +92,9 @@ public boolean isCanceled() {
 
 	@Override
 	public void cancel(final String reason) {
-		cancelReason = reason;
+		cancelReason = reason == null ? "" : reason;
 	}
+
 	@Override
 	public String getCancelReason() {
 		return cancelReason;

src/main/java/org/scijava/module/process/AbstractPreprocessorPlugin.java

@@ -53,7 +53,7 @@ public boolean isCanceled() {
 
 	@Override
 	public void cancel(final String reason) {
-		cancelReason = reason;
+		cancelReason = reason == null ? "" : reason;
 	}
 
 	@Override

src/main/java/org/scijava/ui/AbstractInputHarvesterPlugin.java

@@ -89,7 +89,7 @@ public boolean isCanceled() {
 
 	@Override
 	public void cancel(final String reason) {
-		cancelReason = reason;
+		cancelReason = reason == null ? "" : reason;
 	}
 
 	@Override